How do you know that you are being used?

In a relationship, both use each other. If there is a relationship and it is good, then we give and take. Something consciously, something without thinking at all and then, perhaps, not appreciating (both yours and others).

So the answer to your question in my version is simple.

Think: if you give significantly more than you take, there is an imbalance. You both arranged it somehow. Try to recover. If it doesn't work, break off the relationship. Immediately. - What, for some reason you don't want to vomit immediately? Then you did not take into account something in your estimates, it seems that you get more in this relationship than you calculated. Understand; - is it really, really valuable to you? If yes, everything is in order, see the first paragraph.

Try asking a person for some kind of service, if you are repeatedly denied under any pretext (and you, in turn, have already done a lot for your opponent), then you will definitely be used.

Watch the soul signs of the person with whom you communicate - how much attention he pays to you, how he writes / how he talks to you. Guessing is usually not difficult.

Well, yes. Normal people don't use. Those. if a person uses someone for his own purposes, you are not the only victim.
Be careful.