Edit: upon some reflection, perhaps my stereotypes are outdated and it's not really necessary for even old-money-rich young people to care about fancy watches and wines any more. I mean some base level knowledge is needed, but I have to remind myself that when I think they may have it worse in some respects I'm probably wrong. Why would they restrict themselves to rigid bullshit, if it's not pleasant? Probably the rules are more flexible today. Probably I'm totally blind to whatever the current actual class markers are as they are invisible to us.
The point is that these people don't care about fashion or watches or how much you spend on your clothes. They dress conventionally instead, which is a standard that only changes over the scale of a decade or so. The mistake outsiders make is to think that the point is to follow fashion and buy an expensive watch. That's why they don't fit in - they wear something fashionable.
Also, to defend the idea a bit, the idea of a dress code is exactly that purpose - so that you don't need to worry about what you're wearing or how much it cost or wether someone thinks it's fashionable or not.
That's how I've come to understand it. These wealthy types are not horophiles or sommeliers. They just casually appreciate beautiful craftsmanship & the taste of high quality alcohol- and who doesn't? Just look at how techies love machined metal laptop bodies, and the popularity of craft beer. The reason out-groups are quickly identified is because they cargo-cult the material things.
You can pick up old money relatively easily even if they're in a frumpy grandma sweater, even from a distance.