Speaking of sexing snakes, in this case cornsnakes, by tail shape. I received a couple virtually identical bloodreds in October. They came from Carol, and she received them as a pair, but wasn't told which was which! So when they arrived, I looked at the tails and one was long, thick at the base. One was much shorter, thinner and made a more abrupt transition from cloaca to tail. So I decided the long-tailed one was the male, and the short-tailed one was the female, and named them accordingly. Now these snakes are supposed to breed this season, and I kept thinking, I need to probe one or the other, I need to find out who is really who...And then I thought- wouldn't it be interesting to just put them together for mating and see if I have guessed the sexes correctly, by behaviour? (This would be a not-so-smart move if I was just starting with a random pair of snakes, and I do realize even very experienced breeders can make a mistake, but I was confident enough of the original, pre-Carol source to not doubt I had a M/F pair). So I put them together and the one I had called "male," Ancho, immediately began courting the one I had called "female," Aji. Now they didn't breed, so I still can't be sure Aji is a female, and I'll check by probing if they don't hook up after a few tries, but she was signaling that she might be interested soon, so I doubt that there is going to be a problem!
Again, referring to corns, I started out having a horrible time telling the sex by the tail shape. And I don't think it's super-reliable. More along the lines of a best guess. But I try to think, the male has to have enough room in his tail base to store his hemipenes, which take up a bit or room, so I look for a rather broad base of the tail as my most reliable factor, for a male. If there is a sudden taper after the cloaca, I suspect a female.