Some of our tips here we've learned from painful life lessons, but this particular one we picked up from Law & Order. Criminal suspects aren't too different from cheaters, really - they're just breaking hearts instead of windows. Sometimes we actually wish we had Detective Goren sorting out our love lives so that we didn't have to! Unfortunately this is not the case.
Anyway, the Law & Order Theory of Cheating goes like this: there are two types of suspects, the guilty and the innocent. When cross-examined, the innocent always get angry, and the guilty always get defensive. Now, we're not suggesting that you start grilling her under a hot table lamp here, but if your simple questions start to get wildly defensive responses, it could be time to start turning up the heat.