I think you can feel it by instinct. We girls are blessed with great instinct.
Other than that it can be many things. Him paying attention to what you're saying when the rest don't, him helping you out with both random and important things, or at least offering help, him making sure you get back home safe, and other attentive things like that.
I'm honestly good in theory, but I would probably be as clueless as you in practice. But I've often deluded myself into thinking someone liked me when In fact they didn't, and it happened a lot. Maybe there was a bit of attraction but the majority of times they were going after someone else, close to me, so I may not be the best person to give advice on this. :))

The most sure-fire way to know if a guy likes you is to ask him.
Every guy is different and while some things apply to a majority of them doesn't mean it makes it a sure-fire proof that he likes you. So observe your friends and - if you can't decipher it - ask. Having an open conversation about it the only thing that'll make you sure and prevent misunderstandings.
I know some lies about it as well because they're embarrassed but well - that's the risk you gotta take. Searching for non-verbal hints is a risk as well.

well the most common thing boys do when they like a girl was mostly teasing the girl. like bullying her or being bad to her, mocking and stuff. They also tried to find something to talk to you with no matter what. acting silly and stuff infront of you...(i kind of experienced them...so yeah...since there this guy who keep on texting me saying hi and stuff. then my friend told me that guy actually like me o-o so yeah...) sometimes they also do something that can gain your intention. but not all boys like that though since there are the silent types where they just sit and do nothing except for stealing glance- hope this help-

i don't know much but when he likes u, he will try to tease u or maybe will always find a topic just to talk to u...

Yeah im confused as well.. they said like that but what if that guy just pretending to do those.. *sigh*
Same with me