IIN I can tell that a person is mentally ill after one face to face convo?

I wonder if you're a woman. It's disputable why it's so, but women are generally pretty good at picking up subtle, non-verbal clues about people they deal with, even if they often aren't able to consciously identify those clues. All they know is that they get a "feeling" about somebody. It definitely ain't some woo-woo psychic BS, so they must be seeing and hearing (maybe even smelling) something.

Regardless of whether you're a man or a woman, it's also possible that the "bad experiences" you mention could have left you sensitive to a particular set of subtle clues that suggest a person has psychological issues.

Having said all that, we're all vulnerable to falling into the trap of confirmation bias. Unless you've been keeping some sort of scorecard, it's possible that you had that "feeling" about people who turned out to be well-balanced, but you've unconsciously forgotten about those misses because they aren't consistent with your beliefs about your abilities.