I have what I call the "face" theory.
It's this idea that you can relatively tell what a person is like by just looking at their face. Like "Daddy issues" face, "Meth head" face, "Hentai watching virgin" face, "Gives off the 'I'm wholesome' but secretly has a stash of really vile porn" face. My wife HATES it and feels like it makes me too judgemental. But the first time I met her sister-in-law I thought to myself, "My God, that lady has the 'Incredibly crazy/mentally unstable' face". A few months ago, her sister in law had a crazy mental breakdown that's still ongoing. My wife hates to admit it, but maybe face theory works.