That is a complex thing. Best way to tell is in person, use your instincts. Beside that it is often a pattern of behavior; including (not limited to) not pretending like they (or life) are perfect, reciprocity, empathy. These are the first to come to mind, will let someone else take the baton.

I guess if they “walk their talk”? If their actions follow what they say, I can know they’re genuine either by volition or by default. Some people are genuinely difficult and/or disagreeable!