I only think of living in certain areas and probably being at least the third generation to have a 'Profession'.
So you've more than likely inherited wealth, which has made life easy enough to afford out of school hobbies that are musical, educational etc and expensive, so for me, its being able to indulge in stuff that a close majority wouldn't be able to.
Horse-riding was at one point part of life, then it became exclusive, now depending on where you live, its possible to budget for it.
The markers have changed and will continue to.
The problem is that many people who are doing slightly better than those that they come into contact with/went to school with, or live in an area that has slightly higher property values, think that they are 'proper' middle class and the disdain shown by Politicians/Daily Mail etc, wouldn't be directed at them, but then realise that they are wrong when its too late.
By my definition, I know quite a few people that are MC, my DD's, DP is one. It's tough when you can't give your children what the other side of the family can. They have good propertys and disposable income/savings/shares.
I know loads that think they are, whereas they are just doing well.