If he's really into you, your mere presence can bring about changes or deviations in the way he acts when in you appear. He could be clowning around or laughing hysterically with friends, yet all of a sudden he'll become shy and speechless around you. Alternatively, he could be quiet and reserved most of time, yet suddenly become boisterous or show off the moment you arrive. These things subconsciously show that he wants to impress you.