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)
You notice his ambivalence
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).