Pay attention to his actions and not his words.
Guys are great at creating a disconnect between what they say and what they actually do. This can create confusion if you believe what this guy is telling you. Therefore you should block out all verbal communication and only notice the actions of this person.
Ask friends, family and co-workers to observe how the guy treats you, and how you relate to each other.
Often you are too close to the guy to notice when he is taking advantage of and using you. These people can be valuable tools to help determine the state of the relationship.
Make a list of all the things you do for him and then compile a list of the things he does for you.
Once you finish the lists, compare the length and breadth of the two. If the list of things you do for him is longer and contains more items of significance, then the guy is probably using you, or you are letting yourself be used unnecessarily.
Observe how this person interacts with others.
If he treats his friends and family differently and better than he treats you, then have may be using you. He may not though because if he is using you then he is probably using his friends and family as well. If you believe that he uses his friends and family then he probably is doing the same thing in your relationship.
Invite this guy to do some special favor or important task for you.
If he declines to do it or says he will but does not follow through then he may be using you. Obviously, there are exceptions and if he refuses only once that does not establish a pattern. However, if he repeatedly refuses these requests then the guy is using you.