How to tell if someone has hacked your iPhone 4?

I have an iPhone 4 (not 4S) with iOS 6.
Somebody made a fake Facebook of me and then sent my girlfriend fake iPhone conversation screenshots making it look like I was hitting on another girl. In the fake conversations were some pictures from my phone that I had never sent to anybody (no nudity or anything wrong) and 1 was a picture I had sent to my girlfriend once.
I know my girlfriend didn't send any pictures to anybody because she got really pissed when she got those fake conversations and this could hurt our relationship if I can't prove that it could have been a hacker. (A normal teen not just a professional hacker).
There's a lot more to this story on who we know is behind this, but those are kind of pointless details right now.

Take a help of apps

As long as its not jailbroken then you have nothing to worry about, iphones are really secure.