Monitor online activity, look for changes in appearance (sudden interest in losing weight, dressing up to go on errands, ect.), change in sexual attitudes (suddenly more willing to try new things, or lack of interest), abrupt phone calls (hang-ups, "wrong numbers", late night calls), not being somewhere she was supposed to be, rushing straight to the shower when she gets home, change in communication patterns between you and her (being quiet when normally talkative or vice versa), setting up seperate accounts (checking acct., email, new cell phone), sudden outbursts of affection. Look at credit card statements, bank statements, caller ID and cell phone numbers, internet history, ect.

There are a lot of clues, but you have to look at the whole picture. Just one or two by themselves could just be a coincidence. My advice would be if you really care for her, is to take the opportunity to show her how much she means to you. Make her not want to cheat in the first place. Then talk to her. Express your concerns and tell her why you feel that way.

Good luck and I hope everything works out OK for you.