What do you do if you find out your boyfriend has signed up for one?
Trust him until you can't. (Basically what Matthew Kingsley posted as I was writing this) That's how relationships work. Distrusting him and staying in the relationship anyway is how relationships don't work. If you're at the point of setting a trap, as in User-9683515736950777907's answer, just break it off. Yes, trusting someone can get you hurt. That's what "boyfriend" entails. If you're not willing to get hurt and you just want to "catch" him, just get out now. Neither one of you deserves to be stuck doing that to each other. One last note, I got on one of these apps while in a relationship, just to see what it was all about. I had no interest in meeting anyone to cheat with. Maybe your boyfriend's interest is just the same. ("But what if it's not?" you ask? That's what I'm talking about. Trust him until you can't, and if you can't trust him break it off now.)