You've probably already started some light detective work if you suspect your girl of cheating on you. If she really is cheating, though, you're going to hit a roadblock pretty quickly: you won't be able to find her phone. Even if for some reason she does leave it lying around, you won't be able to get into it because she's changed her passcode.
Sound familiar? Sorry pal, but that's a bad sign. It might not just be her phone, either - it's more than likely that she's changed her password on her computer, and is hiding her diary. You're probably going to start feeling like a parent, especially when you walk in the room and she starts quickly closing browser tabs. Trust us, she's not watching porn, just flirting with someone you don't know.