How do you know if he likes you back...?

So..i'm feeling a lot of things right now. There's this guy at my school who I have liked for a few weeks. He is so adorable..sweet, super smart, and CUUUUUTE ASFFFF. I've never met someone as polite and respectful as him. Whenever I see him, I get butterflies. I get so nervous and I feel the need to impress him and be cool around him LOLLLL. My question is, how do you know if a guy likes you back? I always catch him looking at me when I look over at him. I've never actually talked TO HIM. i get SO NERVOUS. how do i talk to him casually? UGHHH I LIKE HIM SO MUCH