I'm going to make my girlfriend show her texts with another boy
Do you feel that there is someone going on with her and the boy?
I don't know,  I never had a talk with her about it.
As long as you don't think there's a problem, you shouldn't be too nosy with your partner's business.  You need to base your relationship on trust or it won't last.
It's not like I'm going to be stuck with her for life. What's the big deal?
If your girlfriend has not given you a reason to distrust her, you should not consider violating her privacy. Instead, you should talk to her about any concerns that you have.