What are some things i should look for if a guy likes me?

I am 16 yrs old and my brothers friend who is 19 is coming over. I really like him and i know it may seem unlikely because he is 19. But i have been noticing some things. Slight things. Like when i walk into a room i try not to look at him, but a slight glance and he is looking at me (sometimes smiling) and he stayed the night once and that night all of us were sitting on our balcony porch watching a lighting storm, and he was looking out saying we have the best spot for a zombie apocholipse so he was telling us his plan. it was hilarious. the next morning i asked him if he was even paying attention to anything he said, and he was like "Yeah r u kidding! i was talking about zombie stuff, and i was making you laugh a ton!" and i've only talked to him once on FB, but hey it was talking! and he seems to smile at me... But what do you think i should pay more attention to to be absolutely sure? Oh and he is very outgoing if that makes a diff (not like a flirt though just energetic)