We would never presume to be experts on what to do when your boyfriend is cheating on you, and so would never offer advice, just options. One would be to confront him and have a mature conversation where you ask him straightfoward question hoping to get responses in kind.

If that's too boring for you, maybe you should consider giving him a Spyphone for his birthday. It looks like any other cellphone, except when you call it from a special number it connects without alerting its owner, allowing you to eavesdrop on everything they do.

Of course this means that if he is cheating on you, you might end up getting an earful of something you'd rather not hear.

Spyphones [Official Site via Shiny Shiny]