Difference between a natural flirt or a guy actually interested in you.

So there's this guy that I sit next to in class. Every so often he will tap my knee or sit so close that we are shoulder to shoulder. My friend thinks he is flirting with me because he is interested in me, I think he is just a natural flirt. How can I tell the difference?
Difference between a natural flirt or a guy actually interested in you.
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