How do I know if a guy is just shy or not interested?

It’s always how do I know if a man is interested, but how do I know if he isn't interested? I like this guy at work but I am not sure if he is just shy or trying to show he is not interested.
Sometimes when I come in a room, he leaves the room, he makes long eye contact sometimes but when we talk he keeps it really professional. I have caught him staring at me once when I was talking to someone else. Sometimes he just avoids me all together so I just don’t know.

How do I know if a guy is just shy or not interested?
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