@Nuttzy: Arrange whatever evidence you have in your mind in a logical fashion.
Tell her the two of you need to talk. If she says she'd rather not, or she tries to blow you off, tell her the relationship is at stake and if she chooses to part ways at this point in time, she'll have made your decision for you; that decision, of course, being to walk away from the relationship.
Ask her the following question worded in such a manner that it applies to your situation: Are you cheating on me/Have you cheated on me.
Her answer will probably be no. Present exhibit A. If she continues to deny said activity, present exhibits B through whatever. If she continues to deny, and you feel she is lying to you, then she clearly doesn't care about the relationship. Verbalize this to her. If you can't appeal to her mind, appeal to her heart. Tell her how it pains you to have her lie to your face over something that can be inconsequential if only she'd admit to it. If she continues to deny, it's pretty much over. If you can't trust her to answer a question, you can't trust her to do much of anything. She isn't worth being around at that point. If she starts crying, and/or asks you not to be "that way" or tries to save the relationship in some manner, she still cares. If you have no further evidence, and you want to continue with the relationship, its on you to forget about the past. What is done is done, and if there are no further infractions short of pretending what was done wasn't, there isn't really a problem. That is, of course, if she actually did cheat on you. Which we don't know.
If she continues to behave in a suspicious manner, again, don't resort to lowering yourself to spying on her. Just end it. It sounds hard. It isn't. All it takes is a second of courage. Once you've vocalized your decision, the rest is easy.