Experienced snake breeders can usually sex corn snakes by visually checking the tail differences in mature snakes. However, most won't guarantee the sex on sale snakes unless they have either popped the snake at a young age, verified sex with a probe at a later date, or both. I bought a beautiful female tessera as a 3 year old, from a reputable breeder. Prior to breeding season we checked her with a probe and the female turned out to be a male. The breeder said he popped the snake while young, and visually checked it before shipping and was sure it was a female. I admit, looking at the tail I thought it was a female too. If you intend to breed your snakes, have a skilled person or veterinarian probe them.