How can you tell if the boy that likes you is the one?
This maybe too simple an answer, but most truths are just that: simple. Just live and wait. Not like stalker-ish wait at his doorsteps every night or watching his every move but wait and see if he really opens up to you, if he offers you his love. If he really is the one no matter how far apart your heart and physical distance are like the moon and stars, love always has a way of coming together.
Even though it will be painful to endure, even if he isn't the one, you'll become stronger.