How can I tell if she's cheating?

Well. I wouldn't suggest playing mind games, that never ends well. I don't recommend snooping, either. People can be wrong. Especially in high school. Ever play the game telephone in kindergarten. One person says one thing and by the time it makes it around it's something totally different. Just pay attention to what she says and call her out if there are inconsistencies.

If all else fails go with whatever your gut says. Usually, if you feel like something is going on, it typically is. If you don't feel like you can trust her then maybe it's time to move on.