Aside from differences in length, there is no way to visually sex red-tailed boas — at least not reliably. The most widely accepted methods of red-tailed boa sexing are 1) popping, 2) probing, and 3) palpation.
This a procedure which, when performed correctly, opens the snake’s vent and exposes hemipenes (if present). Popping is most safely done on young snakes, and despite the how-to videos on YouTube, should never be attempted by an amateur. Let your vet or an experienced breeder do it for you.
This uses a metal probe by inserting it into the cloaca, pointing it back towards the tip of the tail. Males probe deeper than females because they have extra room for hemipenes. Probing takes a little bit of experience and should only be attempted after you get training from someone with experience, but when done properly it is the best and most accurate method of sexing a boa. Amateur probing risks significantly injuring the snake, so let a vet do it — most vets will probe for just a small fee.
If you don’t have the experience to use a probe or a breeder or vet friend who can sex the boa for you, then you can try the palpation method, also known as the bump test, which is 98% accurate for up to 2-3 year old boas. While holding the snake in one hand, place the thumb of your opposite hand along its spine, starting at the vent and moving downward. Then with light pressure, slide the tip of a finger from the vent towards the tip of the tail. A male snake will feel like there’s a couple of bumps under the skin. Move all the way down the tail to be sure, and perform the bump test more than once, as males will sometimes hold their hemipenes very tight, resulting in a false female result.
For details on red-tailed boa sexing, visit Superior Morphs’ website.