How can you tell if a guy is shy versus not interested?

There's a guy who I've just met, we didn't really talk but he asked our mutual friends to introduce him to me. I catch him lookiing at me sometimes, but it could just be a coincidence. He seems to talk to our mutual friends who are girls no problem and acts really relaxed around them, but is quiet when I talk to him, but maybe it's because we don't really know eachother. How can you tell if someone is shy or just not interested? Or do I not know him well enough to tell?
How can you tell if a guy is shy versus not interested?
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