I asked her if she fuhked this dude and she looked me straight in the face and calmly said no.
That's what my ex-wife did when I confronted her about the guy she was fvcking. She said that she could NEVER date him. One year later she gave birth to her second child who has his last name.
There's a lot of great information in this thread on how to catch cheaters. I'll add mine...
Get yourself a second cell phone. Before you leave, call it with your regular phone via a blocked number and leave it somewhere (hidden in her car, hidden in your house). Drive somewhere and listen.
If her phone is in your name, try and get printouts of the text messages from the past couple of days.
Download a keylogger and install it on her computer. You'll get every keystroke including passwords.
Take those passwords and log into all her 5hit. Check her previous searches on Facebook, her PMs, and check any posts she may have hidden from public view.