How do I know if this shy guy is into me or just a friend I hang out with?

You:

“Hey, shy guy, are you into me?”

(Said while smiling, tilting your head, gently touching his hand with yours and removing it, looking into his eyes dreamily, giving him a wink, etc.)

Him: (a) Either mirrors your actions …

“Yes, I’m into you, too.”

or (b) does not by turning away, cutting off contact, or clarifying his platonic feelings to you directly.

“I’m sorry, I don’t like you like that.”

The short answer is flirting.

Practice it, recognize the non-verbal signs, and always accept the answer you get (i.e., be about consent even if you don’t get the answer you want).

All the best.