How do you know a guy is interested in you? :/

(This is kinda long, I'm sorry)How do you know a guy is interested in you? I'm not used to the whole guy thing unless they actually make their interest obvious. I've only had one boyfriend in total... And that was two years ago. Anyway, I'm 18 (in college) and there's this guy in my class who I never really talk to unless the time arises in group projects. Our first group thing was a couple of weeks ago and we sort of hit it off in a way. He would tease me and pep talk me because I'm extremely shy when it comes to talking in front of people. He even showed complete interest when I told him I'm adopted. After class, he walked with me and we talked about random things. After the presentation (which I totally bombed because my nerves got the better of me), he sort of backed off and wouldn't talk to me until today. He sat next to me, which is abnormal, and he was giggly for some reason and we talked a little and he kept giving me side looks. Then after class, he stayed with me when the elevator was kinda packed. It was only us. Although he could've fit on it, he said he'd simply catch the next. I caught him staring at me but he quickly looked away afterwards. My friends are telling me I should ask him to hand out but I'd rather not risk the embarrassment of getting rejected because I'm not even sure if he's interested in me, you know? What do you guys think...?