This is a good question and one that probably requires an entire thesis to answer, but I would say that in the modern world class is all about behaviour, mannerisms and state of mind. Of course your upbringing is also important. The class you’re born into is what I call your starting class but you can move up or down the social ladder by adopting certain vocabulary and behaviour.
These days it’s not quite as simple as splitting people into lower, middle and upper class. We now see subsets of each of these, so you can have lower middle class, lower upper class, and upper upper class. There’s no middle upper. But let’s say you want to work out whether you’re upper middle class, lower upper or upper upper. There are certain words and phrases that can help determine your social class. For example upper classes will still favour the word ‘lavatory’ over ‘toilet’ making it a pretty good acid test.
Equally if you tend to say ‘pardon’ you’re probably lower class, whereas the upper classes will say ‘sorry’ or even ‘what’ if they’re really posh. And of course you can only ever use the word ‘posh’ ironically. The same is true of ‘common’. If you keep referring to this and that as ‘common’, and not in an ironic or jocular manner, it’s fairly safe to say that you are also fairly common yourself.
“I’ve been to good restaurants all over the country and seen women wearing hats at 7.30pm.”
Dress can be another indicator of class. If a man is wearing brown shoes in the city of London on a Wednesday they’re probably not top tier - as the saying goes “no brown in town”. In the same vein, women should know when to remove their hats. It’s all very well to wear fashion hats that you can buy in high street stores for everyday life, but come 6pm the hat comes off. I’ve been to good restaurants all over the country and seen women wearing hats at 7.30pm. Hats should come off and ladies should be wearing a tiara, if anything, after 6pm.
Perhaps surprisingly, money isn’t such a factor. If you’re upmarket, you shouldn’t really have too much. Money spoils things - it leads to fitted carpets, overstuffed sofas and electric gates - middle class items that higher classes regard as downmarket. The upper class wouldn’t buy those things, they wouldn’t see the point of them. Money doesn’t buy class - the Beckhams and the Kardashians have enormous wealth, but does that make them upper class?