Ask her friends and study their reaction. Something like "has she been with a lot of guys?" If they seem reluctant to answer, then you have your answer. Of course, it could easily backfire and you could end up sleeping alone so you have to ask yourself: "Do you want to sleep with a girl who could be a slut or do you want to sleep alone?"
A woman who has had sex with a million men is not a slut.
Unless she was your girlfriend during the time she did.
I don't hold my woman's past to her. I wouldn't want her to do the same to me. There was a time when I would fark anything that moves. Who am I to call anyone a slut if they are in the same situation that I was in? The only thing I require is that while she is with me, she is truly with me.
Some may call me a fool for this, but I trust my woman. Once that trust is gone, then so will the relationship.