How do you know for sure a boy likes you, if your not around him enough to get all the signals?

Sometimes, guys will make it obvious they're flirting, sometimes they dont. It's really confusing, but its worth it to know. I really wnat to tell the kid i like, i like him....but if he really doesn't have any interest in me once so ever....then i'd rather not ); i need the best possible advice asap. THANKS <3