There are two main accepted methods to sexing a snake. These are called popping and probing. Neither of these should be done if you are not 100% sure of what you're doing, and since you posted this question, I will assume that you are not.
I would never try to use either of these methods on my snakes, since I simply don't have enough experience with it. Your best bet is to take your snake to a vet and have them show you how to do it.
There are a few other sexing techniques which are not 100% reliable and vary from species to species. Male hognoses and cornsnakes will both have longer tails than their female counterparts (if lined up at the vent), though I'm not sure if this hold true for green snakes. This is also a fairly useless method if you only have one snake, or even a whole bunch of snakes all of the same gender.
However, I will repeat what I said earlier, that your best bet if you really want to know the sex is to take your snake to a vet.
females are bigger.