Notice if he lights up when he sees you.
If a guy noticeably brightens up when he sees you, he is excited to be in your presence. There is hardly a more telling sign than a guy who just seems to glow when he notices you. He may start smiling more, laughing a lot or just generally seem more joyful. You know you make a guy happy when he is ecstatic just to be around you.
Observe if he talks a lot around you.
Guys tend not to talk as much as girls, but if he talks a lot around you, especially if he is talking to you, he is excited to see you. This is the case because he will be stimulated by your mere presence and want to be noticed by you. Compare the amount of time he spends talking with you with the amount of time he talks with those around you. If a guy seems to have an endless stream of comments whenever he is around you, he is likely excited to see you.
Detect his interest in knowing more about you.
If a guy asks you a bunch of questions about yourself or seems to care a lot about you, he is probably excited to see you. If he approaches you, looks you in the eye, smiles and just wants to spend time getting to know you, this is a telling sign of his interest and excitement. Notice if he is talking more about himself, or asking you about your life; the latter is a more obvious indicator of interest.
Pick up on the fact that he can't sit still.
If he is truly excited to be around you, he may not be able to sit still. His testosterone is likely flowing, his hormones are reacting and his heart is racing. If he is bouncing around, speaking loudly or driving quickly, he is likely energized by the fact that you are nearby. Pay attention to the way his energy level peaks when you enter a room.
Discern if he wants to hang around you.
If you always find yourself encountering him, he may be deliberately trying to be in places that you will be, and if he is always in your general vicinity at a party or gathering, he may want to be near you. This also applies to groups; if he always ends up around people you hang out with, he likely cannot get enough of your presence.