First off, a meta-answer: Asking "what are non-obvious x" is potentially less useful at capturing less-obvious examples of x than asking for as many distinct examples as possible. I think it is likely that of those who have some [less obvious observations] many will assume they are more obvious than they are for others.
In my case, I have a built-in bias to assume that any piece of knowledge I obtained without apparent effort must also be obvious to most other people.
So, all the examples I can think of, most of which I think are obvious:
Level of agreeableness
I'm now questioning how many of these are generally considered high-class and how many of them I associate with high-class but are more just nerd-culture things that don't entirely generalize.
All of these were off the top of my head in ~20 minutes, quality not guaranteed.
I resisted the temptation to have an LLM generate example ideas, I assume if you wanted LLM answers you would have already gotten them yourself.
I grew up lower middle class and then my dad got a new job and we moved to a far lower CoL area and we ended up in upper middle class of thereabouts. My friend group now is mostly people who grew up upper middle class. Some markers are like whether you went skiing as a child (if you didn't grow up in CO/UT/etc.), whether you did travel sports in middle/high school, whether you went on vacations to places like Cancun as opposed to camping in the woods, whether you went to sleep-away summer camp as a kid. Cruises are a quintessential lower-class vacation. Another one seems to be money, whether you always pay attention to how much things cost. I don't think frugality means lower class, but more like obsessing about prices and couponing etc. Whether you have a monetary safety net from your parents or just a place to stay type of safety net. Lower class people seem to be more into lottery and gambling too.
This isn't politically correct, but weight and eating habits/comfort foods is also a class marker. Whether you like eating very fatty, processed foods. I suspect whether someone likes seafood if they didn't grow up near a body of saltwater is a class marker too, but it's very low n and confounded with being a picky eater.
Class by Paul Fussell is somewhat outdated but a good read, Scott Alexander has a good book review.
Skiing is an interesting one. I never thought about it in those terms since I grew up in Alaska where downhill skiing was relatively accessible (like CO/UT). I also wouldn't be surprised if outdoor activities in general are correlated with class, even when they're not necessarily expensive (e.g. hiking).
skiing
It seems like skiing is a "hereditary" class marker because it's hard to learn how to do it as an adult, and you're probably not going to take your kids skiing unless you yourself were taught as a kid, etc.
Class discrimination is very real, and it is often useful to be able to feign a higher class than one was born into.
This conflict of intent (many people want to be treated as a higher class, but most members of that class don't want to include the "wrong" people as members) is key to undersanding that the markers are subtle and changing. If they were easily noticed and mimicked, they'd become useless for the purpose.
Your social/acquaintanceship circle is a pretty big, pretty hard-to-fake marker. If you can credibly get a mutual acquaintance to introduce/vouch for you, that goes a long way. More impersonal markers come and go, and vary across different locations or sub-groups. They often include speech style (word choice, sentence complexity, diction, not just accent), clothing and comportment (fashions chosen, how recent and good is the haircut, etc.), participation in discussion, with agreement on many topics, but acceptable disagreement showing knowledge of others.
If they were easily noticed and mimicked, they'd become useless for the purpose.
This is a good example of something anti-inductive
Anti-inductive is one of the ways a signal can be costly.
Some class signals are costly simply by... costing money. Designer clothes, expensive accessories.
Other signals are costly by costing time. Could be a lot of time to learn something that doesn't change much afterwards: education, etiquette, accent. Or repeated time spent learning something that keeps changing: fashion.
The most difficult signals to fake are those that require you to have a certain history: school, family connections.
Most class markers evolved to be more complex than JUST money or time. They may include money and time, and they are costly and good components of a signal that one can afford that money or time. But they also generally include other markers of taste, upbringing, or attitudes that can't be bought easily by someone with money and time. Looking down at the crass neveau riche is a pretty well-known behavior of the upper classes.
Especially for time, it's not just whether or not you can spend the time, it's whether you have access to the class-marker-enhancing WAYS to spend that time. Studying fashion in a school won't do it, you need to be embedded in the environment that's collectively picking the fashions for THIS GROUP.
This isn't going to be directly helpful to you because I think you're in the US and this is specifically about the UK, but: the book Watching the English by Kate Fox (which is about many other things besides social class) has a lot to say about social class markers in English society, and (with the obvious disclaimers about the Forer effect) what it says seem to match well with e.g. my and my wife's families. (I would describe mine as lower upper middle class and hers as upper lower middle class, and I kinda hate that it's possible to say such things and have them mean anything.) Again, the US and UK class systems won't be at all identical, but they may have some things in common.
Old SSC post: https://slatestarcodex.com/2016/01/30/staying-classy/
More recent ACX post (about a book from 1983): https://www.astralcodexten.com/p/book-review-fussell-on-class
The appearance of your teeth has class implications; cosmetic tooth treatments are expensive, but common among rich Americans, so the wealthy often have whiter, straighter teeth than do the poor. (AIUI this is not so much a thing in Britain, where cosmetic dentistry is rarer in general).
Larry David knew an audience was not his if they didn't get his joke and Spanish and Caesar using the informal "Tu" form. I can't help but think if you're throwing Shakespeare and polylinguism in your comedy it's implicitly a class signifier. Spanish speakers notwithstanding.
Music is the obvious non-obvious one.
It never occurred to me that my liking the loud heavy metal of Black Sabbath might be a class signifier until much later on in life. Of course they were themselves factory workers from Birmingham, but I suspect that if I had of gone to a more elite school then I'd probably be expected to listen to Art Rock or Prog Rock?
Lemmy of Motorhead remarked about his beloved Beatles:
The Rolling Stones were the mummy's boys—they were all college students from the outskirts of London. ... The Stones made great records, but they were always shit on stage, whereas the Beatles were the gear. ...They were from Liverpool ... a hard, sea-farin' town, all these dockers and sailors around all the time who would beat the piss out of you if you so much as winked at them
Keeping in the UK, apparently there is a divide between Rugby Union against both Rugby League and Soccer.
...rugby union has always been the way that the English ruling classes have lorded it over those uncouth, working-class games...[Rugby] League was a physical, exciting and highly skilled sport played by working men from industrial northern towns.
Tackling rugby union’s superiority complex - David Bowden, Spiked
But elitism hasn’t disappeared altogether. It merely expresses itself in a coded form, whether through Little Britain-style chav-baiting or else in the current ‘rugby is better than football [i.e.Soccer]’ debate. And when I say ‘coded’, you don’t need to be Roland Barthes to deconstruct it. For football, read uncouth working class scum. And that’s why I’ll choose football, warts and all, any day over rugby union.
A dirty tackle on the working classes, - Duleep Allirajah Spiked
I suspect NASCAR vs. Indy Car is a similar divide in American Motorsport. In Australia V8Supercars and Formula One have a similar class dynamic.
Speaking of F1, can any Germans confirm that Michael Schumacher gave off a much more working class vibe than Sebastian Vettel?
It’s not a direct answer, but you might like this thing I made: https://www.lesswrong.com/posts/Yz33koDN5uhSEaB6c/sherlockian-abduction-master-list
@samuelshadrach (currently ratelimited) sent me the following document on the difference between elite and knowledge class social norms. This is not per se about economic class like I'm primarily interested in, and it's more about different social norms than subtle markers, but it's somewhat relevant so I'm linking it here:
Class discrimination is very real, and it is often useful to be able to feign a higher class than one was born into. Some of these class markers are obvious: graduating from an expensive private school, having lots of wealth but not flaunting it, knowing other wealthy/high-status people, etc.
However, some class markers are much less obvious. Given that the vast majority of most people's social lives are spent with people of roughly the same class, it can be very difficult to learn these markers.
I was reminded of this recently, when I befriended someone of a lower-class background than mine but who had recently come into more money. Talking to her, I was struck by just how obvious it was that she had not grown up middle class.
I want to know what markers likely make my upper middle class background obvious to some, but that I never think about because of my relatively homogenous social circles, and more generally class markers that might be hard to notice because of one's background.
Since "upper middle class" can mean a lot of things, here are some facts which may give you a better idea of my background: