If he's sweet to you, if he smiles, if he tells you.
I don't know. That is a tough question.
iam in love with this guy but i do not know if he likes me or some other girl
If a guy likes you but you don't know him he may look at you or walk by you whenever possible.
when he says he likes you.
http://www.wikihow.com/Know-if-a-Guy-Likes-You
Not sure how you expect someone to magically know something.
It is ok to like a guy after you know he likes you. That is perfectly normal and a part of life.
You ask the guy.
don't
his eyes will open wider if he likes it
When he has a penis.
Maybe he wants to seem like the tough to get guy in school? If he does like you then, well, ask him.