It can be difficult, because boys (and girls, very often) in their teens don't have a lot of social skills, and will sometimes be unfriendly or even rude while trying to appear as if they don't care. This poem sums it up exactly:
I am Very Bothered
That's about a 13-year-old, but boys do tend to grow up less quickly than girls, and even in their late teens a lot of them aren't comfortable saying they like you. But if a boy pays you a lot of attention - even if it's not always the kind of attention you want - there is probably some attraction there. At the same time, if someone doesn't have the maturity to let you know he likes you, he is probably too young to be of much interest to you - so it's probably not a good idea to go looking too hard for hopeful signs! If he does like you, and you don't act as if you dislike him, he will find a way to make it clear.