Sexing crested geckos is a fairly straightforward process. Males have a prominent bulge at the base of the tail; female’s don’t. If male, this bulge develops between 18-25g.
Some breeders claim to be able to determine the sex earlier by examining for femoral pores with a magnifying glass or a jeweler’s loupe, but this isn’t always accurate, so sex cannot be confirmed until hemipenes develop (or don’t).