Telling the difference between male and female reptiles is not always easy, and sometimes it is downright impossible by just looking at them. If female and male reptiles have different physical structures and pigmentation, they are said to be "sexually dimorphic." In other words, you can tell a boy reptile from a girl reptile visually.
But dimorphic qualities often do not become visible until sexual maturity is attained. Some examples of reptilian sexual dimorphism are the greatly thinning vertebral scales of male green iguanas, the cloacal spurs of male tegus, and the enlarged, fire-orange head of a male broad-headed skink. The differences may be poorly defined. For instance, a male blue-tongued skink has only a slightly larger head and brighter eye color than the female.
In some cases dimorphic qualities may be virtually lacking, as it is in worm lizards. Except for the males of some species being dramatically larger than the females, crocodilians also show few external sex differences.
Technology has come to the rescue. Using X-rays, the sex of even some traditionally tough-to-tell species can be determined. Besides using X-rays and visual cues to tell the difference between males and females, there are other methods you can use. We will mention some of the other methods below, but will describe two methods termed "probing" and "popping" (herpetocultural terms) here.
Male snakes and lizards have paired copulatory organs called hemipenes. When inverted, these lie in pockets at the base of the tail. Probing is a method of determining whether or not a snake has hemipenial pockets; if yes, it is a male, if not, it is a female. The second method, popping, has been found to be most effective with newly born or newly hatched snakes.
In probing, a lubricated, stainless steel probe of the proper diameter is carefully and gently inserted into the side of the vent of the snake then directed backwards (towards the tail tip) along the interior of the side of the tail. Carefully and gently are the two keywords here. If the snake is a male, and if the lubricated probe is of the correct diameter, the probe will slip the width of 6 to 10 subcaudal scale rows (the scales beneath the tail) into the hemipenial pocket. A female snake lacks the pocket, and the insertion of the probe will be stopped after only 2 to 4 scale rows. If the probe is of an incorrect diameter or if it is pushed too hard, injury to the copulatory organs can occur.
In popping, your thumb is placed a few scale rows behind the baby snake's vent. The thumb is then gently but firmly rolled forward. The pressure of the rolling thumb will expose both hemipenes as they are popped from their sheaths. I strongly urge that you learn both of these delicate sexing methods from an experienced herpetoculturist before trying either yourself.
Boid snakes (pythons and boas) are unique among snakes in having vestiges of rear limbs visible externally at each side of the anal opening. These are referred to as spurs, and are normally much larger proportionately on the males than on the females. In fact, in some of the smaller species, such as the very popular rosy boas, the spurs of the females may be so small that they are all but hidden beneath a small fold of skin. The males of some species and the females of others are larger than the opposite sex. The most reliable method of sexing these snakes (especially the short-tailed species such as ball, Angolan and blood pythons) is by probing. The neonates or hatchlings of small species can be popped. This is more difficult with babies of larger species.
Other snakes lack the spurs of the boids, but most may be reliably sexed by either popping or probing.
Sexing lizards can be more difficult. Most species will autotomize (break off) their tail if you try to pop the hemipenes, and some species are very difficult to sex accurately anyway. Unnatural though it may seem, the sex of some species (most notably the various geckos) can be dictated by incubation temperature. This is termed "TDSD" (temperature dependent sex determination). Yes, this is a true statement. The sex of some reptiles – many lizards, some turtles, and all crocodilians among them – is determined early during incubation by egg temperature rather than genetically.
Radiographing, or X-rays, can effectively determine the sex of many species and, when adult, others may be easily sexed visually. Males of many species of iguanian and agamid lizards have enlarged femoral pores (modified scales beneath the thigh). Although the females may have pores also, they are almost always much smaller. Because of their reproductive morphology, the tailbase of adult male iguanian and agamid lizards is noticeably broader than that of the female. Males of other species (geckos are among these) have enlarged preanal pores (modified scales anterior to the vent) and many also have a "swollen" tailbase. Males of some teiids (tegu relatives) and varanids (monitors) have cloacal spurs (one or more enlarged scales on each side of the vent). Male veiled chameleons have calcars (heel spurs) that are discernable even on hatchlings. Males of many other chameleons (Jackson's and Johnson's among them) have horns or other facial adornment.
Males of many of the sliders, cooters and painted turtles, have elongated foreclaws that are trembled against the head of the female during courtship. Males of most turtle and tortoise species have a thickened and elongated tail, and some have a plastral concavity (a depression in the lower shell) that helps them retain their breeding stance against the rounded carapace of the female. The males of some turtles have a bright red (many American box turtles) or silver-white (Asian black-breasted leaf turtles) iris color that is noticeably different than the duller eye color of the females of the species.
Young crocodilians have no external sexual differences, and how to sex the amphisbaenids (worm lizards) remains unknown.