The only sure way to tell is to "pop" them or probe them, both of which is something that should only be done by someone who really knows what they're doing. Unless you plan to breed him, it's not worth the undue stress and risk of injuring him.

 One distinguishing factor of male and female garter snakes is the size. The female garter snakes grow much bigger than the male garter snakes. Adult males are sixty centimetres long on average while the females are ninety centimetres long on average. Also females tend to be bulkier than the males who are also much more active.