Some pretty simple things that people mention about themselves over time, such as the kind of work they do (or simple statement of the fact that they are from the offending class) makes it easy to make a positive MC guess in many cases.
Of course some of my guesses may have been wrong, or in dispute according to your favoured class definitions. I'm fairly sure that if I mis-accused anyone of being middle class they would have let me know, especially if in some way they were dreadfully offended by it, and if they requested evidence then I would either supply it or retract my accusation.
You and Violentpanda however are accusing me of something rather more unpleasant than being "middle class" and something that I would have some responsibility to alter if it were true. You are doing that and then refusing to back it up with any demonstration as to why it might be true. Life's too short, you say.