Epistemic status: Mostly speculation and anecdotes, feel free to skip to The List once you understand its reason for existence.

tldr: This is a curated list of observable details about a person's appearance that indicate something useful/surprising about them. Ideally, studying this list will be an efficient way to cultivate more insightful observational/abductive abilities, approaching the fictional example of Sherlock Holmes. Please contribute in the comments section after reading the Rules.

Background

Is it possible to develop observational abilities comparable to Sherlock Holmes?

Sir Arthur Conan Doyle's fictional detective has many enviable skills, including mastery of disguise and some expertise at unarmed combat, as well as generally being a genius, but we will focus primarily on his more well known observational power. Though Holmes is often described as a master of logical "deduction," this power is better described as (possibly superhuman) abduction. That is, Holmes perceives tiny details that many people would miss, then constructs explanations for those details. By reasoning through the interacting implications of these explanations, he is able to make inferences that seem impossible to those around him. The final step is actually deductive, but the first two are perhaps more interesting. Holmes' ability to perceive more than others does seem somewhat realistic; it is always possible to actively improve one's situational awareness, at least on a short term basis, simply by focusing on one's surroundings. The trick seems to be the second step, where Holmes is able to work backwards from cause to effect, often leveraging slightly obscure knowledge about a wide variety of topics. 

I spent several of my naive teenage years trying to become more like Holmes. I carefully examined people's shoes (often I actually requested that the shoes be handed over) for numerous features: mud and dirt from walking outside, the apparent price of the shoe, the level of wear and tear, and more specifically the distribution of wear between heel and toe (hoping to distinguish sprinters and joggers), etc. I "read palms," studying the subtle variations between biking and weightlifting calluses. I looked for ink stains and such on sleeves (this works better in fiction than reality). I'm pretty sure I even smelled people. 

None of this worked particularly well. I did come up with some impressive seeming "deductions," but I made so many mistakes that these may have been entirely chance. 

There were various obstacles. First, it is time consuming and slightly awkward to stare at everyone you meet from head to toe. I think there are real tradeoffs here; you have only so much total attention, and by spending more on observing your surroundings, you have less left over to think. Certainly it is not possible to read a textbook at the same time, so practicing your observational techniques comes at a cost. Perhaps it becomes more habitual and easier over time, but I am not convinced it ever comes for free.

Second, the reliability of inferences decays quickly with the number of steps involved. Many of Holmes' most impressive "deductions" come from combining his projected explanations for several details into one cohesive story (perhaps using some of them to rule out alternative explanations for the others) and drawing highly non-obvious, shocking conclusions from this story. In practice, one of the explanations is usually wrong, the entire story is base on false premises, and the conclusions are only shockingly wrong.

Finally, correctly constructing explanations for small details of another person's appearance usually requires shared life experience and context. You might recognize a pin on their backpacking as referencing a particular fandom (how else, but by watching the same show yourself?). You might identify the shoes clipped to that backpack as for climbing (how else, but by going climbing yourself at least once?). You might recognize their accent as Midwestern (how else, but having friends of acquaintances from the Midwest?). So it seems that becoming like Holmes may require accumulating more life experience than others: exploring instead of exploiting, switching careers often, spending time in many different social strata, playing many different sports, rotating your musical and entertainment choices constantly, etc. Perhaps also traveling and living in many different places, though it is perhaps notable that even Holmes confines himself to deep knowledge of life in London. The problem with this strategy is that it is not clearly possible to increase one's life experience far beyond the norm (at least, for one's age group; and it seems that older people run some risk of lacking shared context with younger people). Life experience is somewhat limited bandwidth. I do not believe it is like electrical engineering, where one can learn drastically more about it by reading many textbooks and building circuits instead of passively absorbing basic facts about how the outlets on the wall work and what an HDMI cable is from life experience. You cannot really learn what a climbing shoe looks like by reading (non-picture) books (though @jenn has claimed that reading autobiographies specifically has helped her make some surprising observations). And though it is in principle possible to watch many, many more popular shows than an ordinary person, you cannot watch all of them, and it is basically not worth the time. Also, you'd probably be cluttering your mind with largely useless knowledge. Or as Holmes says (thanks to @Shankar Sivarajan for the reference):

"I consider that a man's brain originally is like a little empty attic, and you have to stock it with such furniture as you choose. A fool takes in all the lumber of every sort that he comes across, so that the knowledge which might be useful to him gets crowded out, or at best is jumbled up with a lot of other things, so that he has difficulty laying his hands upon it. Now the skillful workman is very careful indeed as to what he takes into his brain-attic. He will have nothing but the tools which may help him in doing his work, but of these he has a large assortment, and all in the most perfect order. It is a mistake to think that that little room has elastic walls and can distend to any extent. Depend upon it there comes a time when for every addition of knowledge you forget something that you knew before. It is of the highest importance, therefore, not to have useless facts elbowing out the useful ones."

-Sir Arthur Conan Doyle, A Study in Scarlet

It is this final problem that I hope to address here, by collecting in one place many observable details that indicate useful (or at least impressive sounding) information about others (the explanations). 

Rules

Please add your contributions in the comments. I will impose some basic requirements for what is promoted from the comment section to the post to maintain epistemic rigor and usefulness. 

Rule 1: Do not post useless trivia or anything which is likely to become irrelevant in one year (pop culture, fandom symbols, etc.) unless there is a very compelling reason. Generally if it is not worth memorizing the top 10 examples of the general class that the observation/explanation comes from, do not post it. I am not interested in fandom symbols or transient fashion trends. I am interested in common religious symbology/garb and signs of athletic activities, particularly when those athletic activities imply applicable abilities (like competence in a fight or unusual speed on land or in water). 

Rule 2: I will not promote anything unless I am reasonably confident it works. Either you should have used it yourself in at least a handful of cases (without a significant fraction of false positives) or you should link highly reliable sources that observation -> explanation.  In either case, I will generally require at least one other commenter (possibly myself) to confirm that they have successfully applied the inference in real life. If something is particularly interesting, but I am not confident it is true, I will add it with the label [SPECULATIVE]. 

The List

The list is organized head to toe, as one might scan a person. 

Linguistic cues are listed last, both because they are likely to be encountered last, and because I think they are less worth memorizing. There are simply too many regional accents to keep track of; I'll try to focus on cues with high surprisal. 

Cauliflower Ear -> Combat Sports

Source: Me (Cole Wyeth)

Confidence: High

Relevant background: https://www.nationwidechildrens.org/conditions/cauliflower-ear

I trained MMA and particularly Brazilian Jiu Jitsu for many years. I encountered many people with cauliflower ear, usually former wrestlers or cage fighters. I believe it is common in boxers as well. I do not remember ever encountering anyone with cauliflower ear who did not have a history in combat sports of some variety (a nontrivial observation because I ran frequent intro classes with people who had no martial arts background, as well as many who did). I understand that this is usually a result of repeated trauma to the ear, and though serious swelling usually recedes, after enough instances the ear never really regains it original shape. It has been observed in these circles (slightly tongue-in-cheek) that if you see someone on the street with cauliflower ear, you probably shouldn't mess with them. It is of course also possible that some people have suffered blows to the ear for some other reason, or (if only one ear looks like this) perhaps one very traumatic blow, but I have never observed this. 

Alternative explanations: @DAPuckett apparently has cauliflower ear from cold exposure.

Marks on Neck and Chest -> Violinist

Source: Ana

Confidence: Low [SPECULATIVE]

Obviously, this should be interpreted that these specific marks imply someone might be a violinist. I am not aware of any real life confirmation of this (post if you have observed it). 

Alternative explanations: Apparently cold can cause skin reactions too, though I have some doubts it would cause these precise patterns.

Semicolon Tattoos -> Faced/Faces Mental Health Challenges

Source: @jenn

Confidence level: High

Relevant background: https://en.wikipedia.org/wiki/Project_Semicolon

indicates that someone is struggling with or has overcome severe mental health challenges such as suicidal depression. You see them fairly often if you look for them. i've heard that butterflies and a few other tattoos mean similar things, but you'll run into false positives with any more generic tattoos.

-Jenn

I have also observed this in real life.

Claddagh Rings -> Relationship Status

Source: @jenn 

Confidence level: Mid

Relevant background: https://en.wikipedia.org/wiki/Claddagh_ring#Usage_and_symbolism

learned about this while jewelry shopping recently; it's a ring that looks like a pair of hands holding a heart. it's an irish thing, the finger you wear it on and whether or not it's inverted indicates your relationship status.

Iron Ring -> Canadian Engineer

Source: @jenn 

Confidence level: Mid

Relevant background: https://en.wikipedia.org/wiki/Iron_Ring

In Canada, engineers wear an iron ring on the little finger of their working hand, made from the remains of a bridge that collapsed catastrophically. a decent number of my engineer friends wear the ring.

-Jenn

Apparently not all engineers actually wear their rings, and these are only conferred to undergraduates. 

Black Ring on Middle Finger of Right Hand -> Asexual

Source: Ana

Confidence: Mid, not confirmed

Background: https://en.wikipedia.org/wiki/LGBT_symbols

Apparently this is called an Ace ring. 

Fingertip Marks -> String Instruments

Source: Ana

Confidence: Low [SPECULATIVE]

I am not aware of any real life confirmation of this (post if you have observed it). 

Flagging -> Gay (and, uh... details)

Source: @Ms. Haze

Confidence: Mid

Background: https://en.wikipedia.org/wiki/Handkerchief_code

In-line with lace code is flagging, which has also mostly fallen out of use recently, and is not really done by gay youth these days, but you'll still sometimes see it with older folks. Notably, to my knowledge, it has somewhat less geographic variation in colors than the lace code stuff does (though there still is some).

-Ms. Haze

Personally I don't recommend clicking through that link too much, more details than one would want unless part of the subculture. Still, interesting to know all these handkerchiefs actually mean something instead of just being pretty.