How can I tell if my ball python is a male or female?

Answers:
Ball Pythons are not sexually dimorphic, in other words you can't tell just by looking at them. Male Balls are usually smaller than females, but not always.

The most common and accurate methods are "Popping" and "Probing". Both methods are used to check for the male hemi-penes (the male sexual organ). In popping, pressure is applied at the tail and behind the cloaca to evert the hemipenes or scent glands of the female. This is by far the most accurate method because you can easily tell one from the other if done properly. Popping is usually done on newborn snakes, but can be done on older snakes if the person doing the procedure is experienced and depending on the snake.

Probing is a more common method, although not as accurate. With probing, a metal probe is inserted into the cloaca towards the tail and the distance the probe is inserted will usually tell the sex of the snake. Males typically probe deeper as the probe is inserted into a hemipene, and females will typically probe more shallow. The distance is measured in scales, and it depends on the species of snake. Probing is probably at least 97% accurate.

Both procedures should only be performed by an experienced person such as a vet or breeder. If done improperly, you can damage the snake which can potentially be fatal.

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BYou can't tell from just looking at it. With Ball Pythons, they have to be probed in order to tell. basically a probe is an instrument that's inserted into the snake. Your vet should be able to do this for you, if not contact your local herpetology society and I'm sure one of the members has a set of probes.
If you don't know how, don't experiment, you can harm the snake