For me I think it would be how he/she talks to me, if they ask about/call me without asking for something, at least sometimes, if they include me in their circles and activities, if they help me when I need help ... etc.

I was helping a guy in grad schools a lot and he was calling me all the time pretending to ask about me to introduce his problems for me to help him. I once alluded that I need help to move. He pretended not to hear. Eventually, I realized he was using me. I dropped him and stopped answering his calls at once. I'm usually so naive to know from the beginning.