How can I tell how many men my girlfriend has been with?

Ask her? That's pretty much the only way to get an answer.

The only other thing I could suggest would be to ask her friends or just snoop any of her "friend network" sites and see who she talked to in the past.

^DISCLAIMER:
I don't recommend the latter of the two options because they you can loose her trust and/or she will think youre crazy... you get the picture. Just be honest.

P.S. Also ask yourself the questions.... DO YOU REALLY WANT TO KNOW....and second...How many is too many? You can just ask her to tell you if its more than a certain number you select...if that's just to much than you can make a decision on what you want to do...

GL!!!
-Cuzzinz