Sometimes, when you're convinced that something's up, there's no use skulking around like a private eye anymore. The best thing to do is just to confront her and ask her straight up for the truth. Many times, you'll find out what you want to know. But at other times, she'll have one final trick up her sleeve: complete and other silence. If this is happening often with her, then you might have a good reason to be concerned about what she does behind closed doors.
Silence could mean one of two things, actually: either she stays quiet when you ask her, or she oh-so-casually steers the conversation away into a different direction. Either way, that's a bad sign. No-one wants to lie, especially women, and she'll do anything to not be that person. Even though she's already started one of the biggest lies of all.