It depends on the species. e.g a common garden skink from Australia the females have an orange belly and the males have a light-grey/silver belly
males tails are thicker than females
A boy skink will have a bump underneath the tail a girl will have a closed up hole
one has a penis
size and color
I think by looking at it's scales.
If it has a spot on its bottom it's a boy if not a girl
there are only girl whip tail lizards
hi you tell a boy from a girl lizard by the thickness of the tail the boys tail are more thick than girls
usaully the girl lizards are bigger than the boy.
usally the girl lizard is bigger than the boy
You do not tell if it is a boy or a girl by looking at it's color, the color changing green means the lizard is happy and if it is brown it means it is stressed.
The same with all skinks you cannot really tell there sex apart. If you have a blue tailed skink and you really want to know what sex is you can take it to the vet and they will x-ray it. You don't really need to know the sex unless your planning on breeding it. Its not worth the hassle for such a small little lizard. Hope this answers your question :))
sam, alex, bobby, robin