I find that hearing about his other relationships can be very revealing. Not just with other women, although hearing about the ex-girlfriend/previous sub/first wife can be pretty useful, but also his relationship with his kids, with his parents, with co-workers. One guy that I liked a lot, in retrospect, gave me quite a bit of info about himself that I might have paid attention to - very judgmental about co-workers, not thoughtful of his parents, selfish with the ex-girlfriend. No one thing would have been enough to raise red flags - it wasn't pathological - but when you put them together it was a pattern.