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Class by Paul Fussell is somewhat outdated but a good read, Scott Alexander has a good book review.
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: