How can I find out?

So this is going to sound kind of weird, but I really do mean well. How can I catch my boyfriend watching porn? It’s so easy nowadays to do it as phones have private browsing. I don’t want to catch him to “bust” him or anything. In fact, I don’t mind at all if he’s watching porn. I watch porn myself. It’s just, he won’t be open and honest about it. I just genuinely want to know if he is or isn’t... we are thinking about taking our relationship to the next level and I just want to know this about him. We’ve talked about it, and I’ve asked him up front, and he just says “maybe I do”. I told him I’m fine with it, just would like to know... that way maybe I could know what he’s interested in. I don’t know. I just want to know, I guess I don’t have a main reason. It would just be nice to know that part of him. Is there a way to know? Please no judgement.