In this method, a metal probe is slipped through the vent to check for the presence of a hemipenes, the male copulatory organ. Every male has two hemipenes in the base of the tail. Females will probe from 2 to 4 subcaudals. Males will probe to 7 to 8 while immature, the adults will probe to 10 to 12 scales. When looked at from the side, male tails are generally half as thick at the middle as at the base.
Female tails are usually half as thick a quarter of the tail's length from the base.
While learning, look at adult snakes of known sex before trying it on juveniles and adults of unknown sex. Male boa constrictors have spurs on either side of the vent that are significantly larger than those of the females. Breeding Boas requires closely copying their natural conditions. This includes a seasonal day night light cycle, winter cooling, misting, and many other factors. Timing is crucial.