There are a few physical differences between male and female red tailed boa constrictors, but the significant attribute used to specify their gender is the sex organs. Male red tail boas have hemipenes: inverted double penii that are long and dictate the length of the tail. Female red tail boas have hemipenial homologues which are shorter pockets equivalent to the hemipenes. Probing and popping are two methods of discovering which sex organ your red tail boa has.
Restrain the red tailed boa by having someone else hold it and restrain its agitation, or by placing it in a plastic restraining tube. Probing is not harmful to a boa but it is agitating. This method is not unsanitary as the sex organs of red tailed boas are inverted extensions of their exterior, just as clean as the rest of their body.
Place a probe that is of significant size (as large in diameter as can possibly fit into the cloaca of a male boa), slender, smooth and lubricated, into the cloaca of the snake. The cloaca is the opening at the base of the tail.
Push the probe against the posterior wall as far as it will go. The probe in a red tailed boa reaches commonly 8-9 subcaudal scales long. The probe in a female will often go only 1-2 subcaudal scales down the length of the tail. This has to do with the size of the hemipenes and hemipenial homologues. A probe that is too slender may puncture one of the pockets in a female and lead to a mistaken sexing.
Pull the probe out gently and reinsert it, directing the probe against the opposite side of the posterior wall. There are two pockets in both male and female red tail boa sex organs, slightly to either side of each other. Probe both to ensure a more accurate sexing. A female will often have two different lengths of hemipenial homologues and male hemipenes are similarly long.
Restrain the red tailed boa. This method is less invasive but equally agitating. This method can only assure correct sexes when executed within the first few weeks of the red tailed boa's life, as the male gains muscular control and prevents the popping of his sex organs.
Hold the base of the tail with a thumb and slide the anal scale to the side.
Put pressure with your other hand on the underside of the tail at its base opposite your thumb.
Gently rock the bottom of the tail up towards the exposed anal vent to observe the characteristics of the released organs. This can only happen in a relaxed red tailed boa. The male hemipenes will pop out and appear as small pink or red rods. The female glands will appear and will never have a red blood vessel visible.
Run your thumb with pressure from the end of the body of a young snake down past the anal vent. With a red tailed boa that is relaxed and young enough, you will feel a popping similar to a rubber band snap. A female red tailed boa will have no reaction.
Observe the length of the red tailed boa's tail. A female's tail tapers directly from her anal vent whereas the male's must taper later to make up for his longer sex organs. As a result the male's tail is much longer.
Notice the behaviour of your snake. Two male's together may fight. However, a lack of confrontation does not determine the sex of a red tailed boa. Dominance behaviour often seems like copulation and therefore perceived actions can rarely be used to determine gender.
Detect detailed differences such as small sperm deposits in the side of a water dish or thin shedded skin from hemipenes. A female in general will be smaller than a male and the pelvic spurs that remain are rarely intact enough to use as suitable gender differentiation.
Consult your veterinarian before proceeding with any procedure. Perform sexing as early as possible to ensure accuracy.
Restrain the large adult red tail boa adequately during any sexing procedure as they can be agitated and are large creatures.