Most experienced keepers can tell the sex of a snake by just looking at the underside of the tail most of the time. The shape just rearward of the vent(cloaca), and the length of the tail in relation to the snake as well. in young snakes they are easily sexed by a technique known as "popping", and older ones with much more muscle tone by "probing" with a thin lubricated stainless steel rod.
Here is a link with pics and videos to explain in detail exactly how to do this. This is extremely easy to do, but if done incorrectly, you can easily permanently injure the snake. I have even known of some to even kill the snakes from excessive crushing trauma, so check out these techniques carefully in this link, and be confident that you understand the process very well before attempting to do this. I have shown many people how to do this over the years, and it is really very simple and straight-forward once you know how to do it, and why.
good luck!, and here is the link
~Doug
sexing snakes
-----
"a snake in the grass is a GOOD thing" 
my website -serpentinespecialties.webs.com