If you are in 5th grade or below, he will probably pinch you or throw things at you.
If you are between 5th grade and 8th grade, he will probably panic and clam up if you try to talk to him.
If you are in high school or college, he may have finally mastered the skill of asking a girl out.
Obviously, things are much more complicated than this. Some guys are Casanovas by the age of 10, and others are still too scared to even approach a girl they like until they are 25.
In general, there are some signs you can look for. Does he fix his hair or do other peacock-like primping when you are around? Does he try to make ways to hang out with you such as walking with you between classes or sitting near you at lunch? Has he talked to his friends or yours to try to pump them for info about you? All of these are good signs.
If you happen to be close to any of his buddies, maybe you can ask them if he has the hots for anyone (be subtle though, you don't want them telling him you were snooping).
Really, the best way to know if a guy likes you is if he just plain asks you out. You can spend all the time in the world analyzing if he blinked at you a certain way or if your horoscopes match up, but most guys are pretty simple creatures. If they like you, they will probably want to hang out with you!