How can you tell if a guy who has a girlfriend likes you?

Well I've been friends with this guy for awhile, and he has a girlfriend but he acts like he likes me. He's super sweet and is always complimenting me because I like to sing. And he is constantly telling me how good I am. And he calls me all these nick names and stuff. But like I was just wondering what are some signs that a guy who is in a relationship could send someone he likes? Or maybe a better way to word that is how can you tell if a guy who has a girlfriend likes you? :) thanks !
How can you tell if a guy who has a girlfriend likes you?
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