tl;dr: By default, iPhone saves the date and time of every location you visit, and shows them on a map. Want to know if a girl is cheating? This will reveal it almost every time.

Body: On your iPhone, visit Settings->Privacy->Location Services->System Services->Frequent Locations

This contains every location your phone has visited for the past 2 months, mapped and ranked in order of frequency. This feature is turned on by default, but awareness of it in the general population is (as of now) extremely low. If you are dating a girl with an Android phone, I can't help you. But then again, in the United States, girls with Android phones are particularly sketch.

Where you physically travel to tells a lot. Was your girl at a restaurant, bar or nightclub that she didn't mention to you? Was she in a residential neighborhood at 2 in the morning? (Or even 2 in the afternoon). Once you catch her in a lie about where she was, it becomes easier to coax the full truth out of her.

The feature is particularly useful after a girl has traveled, when you may have found yourself victim to the classic "I'm going to bed" videochat, typically followed by a change of outfits and night on the town.

Because AWALT, this is a very useful thing to check from time to time. And if you are up to no good, you should probably turn this feature off lest it be used against you.

Lessons Learned: It's hard to tell if a girl is cheating by looking at her messaging history, because girls are savvy enough to delete them / change the names involved. Where they have been often tells an extremely interesting story.