On the other side of the opinion of "confronting her now with limited evidence/talking it out etc"
You could use hidden cameras, keyloggers, the whole hog, there is even discreet software you can put on her phone that will text you whatever messages she gets, either via text messaging (but this would show up on her bill) or via bluetooth, but obviously your phone has to be in range to pick this up.
You need concrete evidence before confronting her, which you will only get via more investigation, once you have it, you can confidently approach her with the evidence to break it off first, knowing there is no doubt she is cheating.
This is coming from someone who doesn't trust anyone, and above all women

Yes I am bitter and twisted
edit: A quite easy way of finding out if she is indeed lying, if you have a good mate that she doesn't really know well, or what car he may drive, then when she says "I'll be at my parents" or "I'm going to the pub with friends after work" then get him to follow her and get him to ring you if she deviates from what she has said.