How to know if she is cheating on me

Do you think that she is?. If so, what brings you to that feeling?

OK first of all is it online? Or in person.. I found my boyfriend cheating on me online and I found this out by (yes I'm ashamed) by looking throgh his chat log on msn but I had this gut feeling something was going on because he was distant from me sexually and generally.. he never opened his msn chats while I was in the room which you no worried me and after a month of this weird behaviour I dicieded to find out by searching.. I hope your girlfriend isn't chaeting on you and I wish you the best of luck.. if its online contact me because I might be able to help

Why do you think she is

You find her out dating someone else is a fairly good sign. Not nearly enough info, why do you think she is ?