I have had many queries regarding how to sex hognose snakes. Here I will deal with the nasicus group, the western hognose snakes.
The pictures used here are of neonate Mexican Hognose Snakes, Heterodon nasicus kennerlyi. The snakes are about 4-4.5" long. On adults sexing is easier, but it is readily evident even on hatchlings once you get used to it.
The picture on the left is of a female, on the right is the male. The female has a much shorter tail, somewhat stubbier in appearance. On the male the tail is much longer and not stubby.
From the side view on the ventral surface, the female may have a "dip" behind the cloaca, as is evident in the picture on the right. On males the tail goes straight back. This is noticeable on many colubrid snakes.
Owing to the small size of the tails of neonate nasicus, and the ease with which they can be visually sexed, I do not recommend either "popping" or probing as a method of sexing on them.