It costs money but you can get someones outgoing and incoming calls from their cell phone. Get my email from Jack if ya want. try googling reverse cell phone....that could lead you in the right direction
I'll do the P.I. work for you, just wait a sec........yup she's cheating on you. Sorry about the news, but at least you didn't have to pay someone for it.
Do you have access to the phone or not? Should take about 20 seconds to get into if you can get to the phone, but depends on what model. PDA/Blackberry types are pretty easy.