I guess, by asking? And then you discern reality to the best of your abilities comparing your knowledge of the person and what they’ve said. And if you get a defensive answer like “that’s my business, how dare you, yap yap yap!” (or something like “it’s not like a keep a tally!”) then you know what’s up.

I just told my partner before we even slept together, which I feel is the least one can do (in many parts of the world having sex is sort of a point of no return, not just something fun and casual like a dap between co-workers). And what I told her wasn’t pretty but that’s how it is, and she could’ve just told me she was disgusted and couldn’t continue seeing me as it was her prerogative and an understandable decision to make, she just didn’t.