If he asks you out.
OR ... Since we're living in the 21st Century ... If YOU ask HIM out, and he accepts.
I like this one boy at my school, and I'm not 100% sure if he likes me back. What are some more-than-the-obvious ways that he likes me back?
Another one...
You could ASK him! That would give you your definite answer! Or... You could get a friend to ask him if he likes you!