There are a few ways to do it ..
One of them is "poping them" .. This , in my oppinion should only be done on neonates. Basically you put a bit of presure on the back of the spinal cord (over the claoca .. and push gently on the scales right after the cloaca where the hemipenes would be if it was a male .. then their privates will pop! and males have two hemipenes and females don't .. DON"T DO THIS UNLESS u have already seen someone do it or know how .. u could seriously hurt your snake if you don't know what ure doing ..
Most commonly used manner is probing them .. You insert a probe of the right size (in my oppinion), probing should be done on a snake that is older than 6 month if not a year ... You insert it in the cloaca in the directions of the tail. Find the hemipenes and it goes about 7 - 10 sub caudal scales (I think there called like that) down towards the tail . Thats a male .. if it only goes down about 3 scales its a femal .. How deep varies on the species being probed .. AGAIN DON"T probe a snake unless u trully know what u are doing .. this could be very dangerous for the snake if u were to make a bad move...
Alos on larger boids .. u can use the hind spurs as a good indicator .. large spurs generally means male, small could be femae..
Also by judging the tail .. if youve seen many, a male has bumps under the tail females don't ... hard to know .. only an experienced eye can tell.
I hope it helped...
1.3 Coastals 6.6 Jungles
3.4 West Papuan 1.0 Bred'ls
1.1 Yellow condas 0.1 Sebea
**looking for female Bredl's python**