It is a difficult question to answer because I suspect there are many aspects that either include or exclude someone from it. A bit like asking what is art, but knowing it when you see it, or knowing what you like, but finding a full definition is nigh impossible.
But if you are one of those who finds they have to look for an employer in order to get an income to pay the bills, yet have to compete against many others doing the same thing so get rejected time and again before finding a niche, one that probably wasn't the one who really wanted anyway. Or if you didn't have the confidence to take a risk bankrupting yourself on some business scheme. Or if you didn't get straight in straight away into a job in a company on a well above average salary because you knew someone who'd put a good word in, and this sort of advantage gave you the confidence to go for things you probably weren't that good at, believing that this was normal and right; and if you weren't baffled as to why the masses were moaning at your salary compared to theirs, nor understand why they didn't just do the same as you; and if you didn't go around blaming their attitude on envy, then you are probably working class.