In my opinion here’s the difference between helping someone and being used by someone-
If you would be getting paid the going rate for your services would the person have acted the same way.
Here’s an example that happened recently-
Two people called me and asked for help with their Regents/ Finals.
I gave them both an hour and told them beforehand to look over the material and bring any questions they have at that time.
Person one comes with a list of questions, watches how I solve the problems, took notes for future review and organized all my papers and his before he left.
Person two comes in and starts doing problems until he gets stuck, asks me how to do it, says it’s too hard, he’s giving up since there’s no way he’ll pass anyway. Doesn’t bother to take the notes where I showed how to do the problem since he’ll be repeating the course over the summer.
Person two was using me, had he been paying the going rate of $60 to $80 per hour (or whatever it is), he would have made it is business to take it more seriously. Person one, on the other hand, I was very happy to help out.
(I’m not sure how this helps the OP but I think it answers the question of how to tell the difference between being used and doing a favor.)