How do you know if a guy likes you or not?

There is this guy at school in my band class....he stares at me while he is pretending to look down playing the piano or looking at other ppl and then smiles....i've always cathed him staring....he goes and sits close to me and takes his phone out and starts txting or sumting....when im with my friends and he's kinda nxt to us with his other friends...he plays playful with them and little kids