How can I tell if we are friends with benefits or if he is just using me?

He and I are in the same graduate program. I finally got to know him recently because we were in a seminar together last semester and we hooked up at an end of semester party. I found out later that he had a girlfriend and they were on the verge of hooking up. I was upset and luckily was able to avoid him for the time that I remained on campus. My friend is in cl with him and found out that he is now single. I also ran into him a couple of times on campus and he was overly friendly. I have a soft spot for him and we hooked up again this past weekend. We have mutual friends and more often than not, end up at the same party, function, etc. How can I tell if our hookups are FWB or if he is just using me?