When you get together with someone, listen for terrible things he's done in the past. Know that if you're lucky, if the relationship lasts, he will do something at least that bad to you.
I do take into account circumstances, regrets, and any changes he's made to his life. But I find this rule is pretty reliable.
The guy who threw someone's possessions out a car window at speed, he was a bit of a dick with me. The guy who hit his girlfriend (and was deeply deeply ashamed of it) didn't hit me, but he did get v angry somewhat randomly. And so on.
Does this rule work for others?