How do you tell a boy blue tailed skink from a girl blue tailed skink?

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 :))