It would depend on the person and how badly they had screwed me over in the past. If I lacked the history with them to form an opinion based on personal experience with the person, then I would look to their behavior in general- are they generally honest, sincere, forthright in their dealings with other people? Or do they screw people over and hurt them or rip them off and try to justify it, and are they full of stories about people who've done them wrong, which sound a little too bad to be true, and which aren't consistent (the same story is told differently, or changes, or when confronted with a fact, they start changing the story, ect) over time?
I have learned that you don't have to sleep with someone to tell what sort of a lover they'll be, and you don't have to get involved to find out if they're a lemon or not. Just sit back and watch them in action with other people, and confront or question them on occasion if there is something that doesn't add up or that you don't understand, and see how they ract. Unfortunately, it is easy to fall into the trap of wishful thinking and longing for acceptance, and overlook all this, but it's there to see in you look for it.