What are the signs that my boyfriend is only using you?

Whatever he may be using you for, whether it is sex, money, popularity, an ego boost, etc., you'll know by his interest in you when any of that is absent.

If you aren't having sex with him, funding his life, making him friends, praising him or whatever it may be, and he has a sudden lack of interest to be around you, or a lack of interest in you in general, there's a good chance he's only using you. That is especially if this is consistent in his attitude.