He doesn't forget the stuff you tell him
If he has a crush on you, a shy guy will remember way more than you expect him to, about you. Because he likes you! You stick out to him.
He lights up when he sees you (and doesn't with anyone else)
Shy guys have a tendency to look timid - even despondent at times. But when an introverted dude perks up at the sight of you, maybe treat it as an insight into the way he feels. He might even immediately try to cover it up, but unhindered enthusiasm at the sight of you (and not that many other people) is a pretty clear indicator that shy guy's into you.
You notice his ambivalence
A shy guy will have a sort of "push-pull" dynamic with his body where he simultaneously will lean toward you but also frequently back away. This is a natural response to anxiety, which a timid guy might have more on than the average guy - especially if he's attracted to you on a physical and emotional level.
Even if he has trouble carrying on a conversation, he'll try to say hi to you
Shy dudes might not be expert conversationalists, but if they like you, they'll go out of their way to at least say hello (and it will be sort of awkward).