He stares at you... (Your face, not just your other body parts.)
He becomes clumsy, tongue-tied, or awkward around you.
He remembers small details about you that other people don't remember.
He touches you -- even low-key subtle touching.
He sticks up for you.
He behaves in a more cool, mature way when you're around.
He shares a playlist with you. (Very intimate!)
He asks real questions about your life. (Other than just "How's it going?)
He asks if you want to "hang out" casually. (If he really likes you, he may
be nervous to actually ask for a DATE!)
He asks questions about you to your friends.
He tries to find out who you like: is it someone else? Or maybe it's him?
He says your name a lot when you talk with him.
He calls you, texts you, or chats with you on Facebook more than he used to.
He talks about other girls around you, trying to make you jealous.
He refers to you by a nickname (even a slightly teasing one).He laughs at your jokes... even the ones that aren't very funny.
When he jokes about something, he looks at you first to see your reaction.
He talks as if you're a couple, makes flirty references to you, jokes about dating you, etc.
He compliments the way you look.