Watch your friend for changes in his health. These include bloodshot eyes, a runny nose or nose bleeds. If these symptoms last longer than a normal cold, then look for other signs, such as weight loss.
Pay attention to his attitude. If he suddenly seems irritable, or tired and depressed all time, or quits eating and stays up all night, then cocaine could be involved.
Notice changes in his social life. If he starts hanging out with different people, or loses interest in his friends entirely, then he may be using cocaine. This may also be true if he loses interest in a favorite activity or needs money on a regular basis.
Watch for signs of white powder on his nose or face.