You will have to probe the snake. females will usually probe about three scales but can go to five or six scales deep the males up to ten scales usually about seven scales.
You will need to find a experinced herper to do this.
Probing is just what it says, using the correct sized probe an some lube,people have different prefs, you put the probe in the vent and gently insert it in going away from the body towards the tail. It will stop when its reached the end,pull it out and count how far it went in against the tail scales.
another way is to "pop" the snake. This too you'll need some one with experience. This involves pushingand rolling your thumb from about half way down the tail up to the vent. this will push out the hemipenes in males and musk gland in females.
This realy only works well on young snakes.
I hope I did an ok job explaining, I'm much better showing than tring to think typ and explain.
Piers
guppyranch@uniserve.com