I believe The only two ways there is to properly sex a snakes is,one is popping the snake and the other is probing ,the popping is the preferred way to go when you are a beginner and still then you should have some one showing you,The other method probing should only be done by experts or someone who really know what they are doing/herp vet. I am no expert but i am pretty sure i supplied a decent enough answer but still wait for others to reply i am sure you will get more detailed answers from them.Good luck.
Here is a video made by Ralph davis, showing how to properly pop and probe snakes ,this help me loads and hopefully it can help you as well.(its to parts)
http://www.youtube.com/watch?v=wMqN402TiVg