Hi Joe -
Here's a site that describes the what, where, how, and why:
www.vpi.com/9VPITipsAndTechs/DeterminingTheSexOfSnakes/DeterminingTheSexOfSnakes.htm
Here's a link to Kathy Love's short videos on how to probe snakes:
www.corn-utopia.com/Corn Utopia on the Web/- Sexing corn snakes - intro to vid clips.htm
Here's a site that demonstrates how to evert snake hemipenes if you're interested in that method:
http://www.[url ban]/sexingtutorial.htm
When all is said and done, however, it's nice to have someone demonstrate in person the first time. Besides what others have already mentioned, consider checking out a local herpetological society meeting in your area for help.
And when it comes time to do it for yourself, it is always nice to practice on some big critter that you can use a big, blunt probe on instead of a small hatchling with the necessarily more sharply-pointed small probes (Jab one on your wrist to get an idea). Good luck!
-Joan
Oh yeah, then you have to interpret your results. So you can come back here and ask everyone, "Hey, what does it mean when it probes 5 subcaudals?" 