One of the most obvious signs that a guy likes you is when he looks for opportunities to see or talk to you. A good example of this would be finding him waiting at your locker in school or when you catch him staring at you severally.
He's nervous and self-conscious about himself whenever he's around you. (Even for the outgoing guys). He might also try to impress you, or he might be flirty. He also looks for chances to talk to you, like maybe in class. But the most positive sign is when he tends to glance at you alot when you are no noticing/ you happen to see him looking at you.