It can take as much as 9 months before it is possible to accurately sex crested geckos. Best to keep them separated till the new one is at least 9 months old. A female crested gecko with tail should be at least 35g before breeding and a year old, or older.

Only other way to visually sex a crested gecko is by looking at the rows of pores along the underthigh and before the vent. If I recall correctly males have over 2 rows of pores..which often just look like little holes in the scales, one per scale. Females have these pores as well but only have one or two rows.
-----
PHLdyPayne