http://webspinners.com/coloherp/cb-news/archive/careshts/boacare.php said:
SEXING AND BREEDING
A snake's sex can be determined by several methods:
Neonates: Very often a male neonate can be manually everted by placing one thumb on the back of the tail and rolling the other towards the cloaca. The hemipenes will evert easily, but this should only be done by experienced individuals, as damage from excess pressure is possible.
Probing: This is the most reliable way, but it should only be done by an experienced person. 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.
A simple visual examination of the tail's length and thickness: The hemipenes make the tails of males longer and thicker than the tails of females of the same size. 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.
Spurs: Male boa constrictors have spurs on either side of the vent that are significantly larger than those of the females.
