Probing can be very accurate (99%) as long as the person knows what he/she is doing and sticks to some ground rules:
1/ Never probe a snake that is close to sloughing, the sperm plugs in males can restrict penetration of the probe and male could be mistaken for a female a female.
2/ Use the right size probe.
3/ Keep the probe in a glass of warm water (no one likes a piece of cold steel showed up their cloaca) and lubricate it before inserting.
4/ If you think it's a female, probe both sides to be sure, no need to probe both sides if the probe goes long way in.
5/ If in doubt, repeat the probing straight after the next slough.
6/ Handle the snake for a while before probing, touching at around the cloaca with (warm) fingers, it helps the snake to relax.
7/ Best to have someone to assist you with restraining the snake, so you can concentrate on the probing job.
Of course, steady hands, patience and focus is needed too, it's not something you do in hurry or when you having a blond day.
cheers
Michael