I am suspicious my fiance is cheating...text messages?

I believe that my fiance is cheating on me. She texts one person all the time and is on the verge of breaking up with me. If i could be definite, i would check her texts, but she has that phone on her 24/7. And when she is done texting i see her delete them. The phone is under my name (Tmobile) and i can see the numbers she is texting, but not the message. Is there any way i can get a print out or a way to visibly see the texts not through the phone? Is that even possible?