If you really like a guy should you tell him before he moves?

Don't seem desperate. Just say that you'd like to keep in touch after he moves, then e-mail or text him to follow up. That will give you both a chance to figure out if you want to maintain a long distance relationship. If the person in question is moving more than 25 miles away, you are probably better off keeping your feelings to yourself, because it is extremely difficult to have a long distance relationship. There is nothing wrong with letting them know that you think that they are a nice person, with good qualities. What purpose do you want to achieve by telling them that you like them?