What can I do when I know my boyfriend is talking to my friend behind my back?

I found this out by checking his text messages. She sent want to him that said 'sounds good baby' and he wrote back 'awesome'. I don't know what sounds good and I don't want to ask him because he'll know I was snooping. I want to catch him in the middle of something so he can't turn it around on me. Am I being paranoid? Does it sound like he's cheating on me?