Im my experience, where a guy feels he cant trust the girl enough that he has to go through her phone.......... it has always turned out that the guy was the one who ended up being the one to play away from home

Clearly you dont trust your girlfriend, talking to the opposite sex on text messages is always going to look odd to the boyf / girlf, its human nature to get jealous, but if you truly trust your partner then I dont see that it's an issue, I would ask yourself why you feel she would decieve you like that and why you dont trust her?

I used to get the arse with my ex going through my phone every time I wasn't in the room, so to stop the arguments I just deleted anything I knew he'd kick up about as it came thru, then he could look through my phone all he wanted. Now she knows you're checking up on her, she'll be more careful where she leaves her phone and what messages she leaves in it, then you'll never know the truth about what's going on, catch 22 really