superdave said:Seriously, just want to know what you guys think.
Are you afraid that she used to be thin? Sorry, couldn't resist.

I think you need to watch Clerks. Some good info on past behavior of girlfriends and general sluttiness.
Aside from that, what are you bothered about? If you are in a 'new' relationship, there will still be plenty of things you really can't ask about. But, if you have been with her for a couple of months, ask away.
If you are seriously thinking about being with somebody for any length of time, you have to FEEL comfortable. And feelings aren't logical, and they aren't right or wrong. So don't apply logic to this. And this feeling is for everything: her family, her cat, her smell, her mannerisms, the number of cocks she has sucked, the number of coochies she's licked, etc.
But, the sooner you find out, the better. Because if you find yourself falling for a girl, and you later find out she is not someone you want to be associated with...that is trouble and pain.
Good luck.