How can you tell if a quiet, shy guy likes you?

There's this really really quiet guy that I've liked for a while and I've tried sparking some conversations with him about schoolwork and tests. Although he's normally shy and doesn't engage much when others talk to him, he's always engaged in our conversations one-to-one and will keep asking me questions too. Even in a group setting if I talk to just him, he'll engage in the conversation. I've known him for a while now and we've always had random moments of him looking at me in the hallways and smiling a little. But recently when I walk past him, he's smiling really hard as he's looking at me and of course, I smile back (but we're not exactly on waving terms). And then there's also times where we walk past each-other and he doesn't look at me.

How can you tell if a quiet, shy guy likes you?
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