I'm yet to hear of anyone who probes with 100% accuracy. I know of snakes missexed by probing by all of the well known breeders. Some get pretty close to 100% but surprisingly few, and some of the ones who tout themselves as the best are little better than Gary's method.
The only absolutely positive way to tell for sure is to breed them. The one which lays eggs or gives birth is the girl, and the other is the boy.
Some species are extremely easy to sex by tail shape, including many of the small elapids. Many species can be visually sexed much more easily than by probing (such as Bardicks
). In these species, the visual difference is extremely easy to see.
Males have longer tails on average, and in some species it is very obvious. In most species of pythons (which I assume is all you are interested in) the average difference is small and there is a lot of overlap between the sexes, so measuring tails won't help you.
Here is a picture of a male and female Little Whipsnake. You can see the male's tail is far longer, and has obvious hemipene bulges. The sexual differences in this species are unmissable.
Male:
Female: