To sex them take a quick look at both lizards.
Head size: This is perhaps the attribute that is most commonly mentioned when sexing BT's. Truth be told, this is also one of the more unreliable physical characteristics. Males do generally have a wider head than females, however, the differences between various animals and species are usually to slight to be accurate.
Body shape: Again, helpful, but not an accurate sexing method in and of itself. Males tend to have straighter sides, with females slightly outward bowing at their middles. Again, though, older, heavier males will appear to be females by this method.
Hemi-penile bulges: Male blue-tongues are equipped with two separate sex organs. One on each side of its body. These are located in the base of the tail in pockets. Because of this, it is possible to see the bulges created by the hemi-penises. You will notice the difference in the tail base of the two animals below. The picture above is a male, and the picture below is the female. This feature is more easily displayed during the mating season as the sexual organs swell, and is not visible in baby and juvenile BT's until they reach sexual maturity.
Eye color: Yes, eye color can be used to determine the sex of a blue-tongued skink. Males have a more intense orange colored eye, while females are more brown to brownish orange by comparison.Again male is shown above and female below.
You should see some activity at anytime after the winter season when temps are raised and feeding is back to normal.The male will begin to follow the female around the enclosure, with his nose behind the females back legs. Females will normally react by wiggling and twitching their tails when confronted with a male. He will then attempt to position himself, attempting to secure a hold on her by biting her side, near the front leg. If he is successful in restraining her, the male will then attempt cause her to lift her tail base, both by trying to push her side up, and by scratching the top of her tail with his hind leg. If she is reluctant, he will eventually break the hold, and usually will rest before attempting again. If on the other hand, she allows this, brief copulation insues.
You can keep the pregnant female outdoors or seperate her from the male in her own enclosure to give birth to the babies.IMO the latter choice would be recomended so as you could keep a closer eye on any offspring.
Sexing blueys is a pretty hard task,i have spoke to many breeders & most say it is near impossable till they are adults,some tried popping hemipenes at birth only to damage the reproductive organs.The easiest way i have found sofar is to look at head size & body size,the females tend to be large in body with littler heads,the males small & big boof heads.The old tale of the pink belly for girls is just that as my male is the only one with a pink belly.Breeding time is a good way to determan sexes as males will fight. 
