Leopard geckos can be sexed after they have grown 5-6 inches long. The most reliable method of leopard gecko sexing is inspecting the areas before and after the vent.

• Males have pre-anal pores, which looks like a small line of dots across the underside of the back legs in front of the vent. They also have two oval bulges behind the vent – these indicate hemipenes.
• Females do not have pores or hemipenal bulges.

Males and females should not be housed together unless you intend to breed them. Even then, the arrangement should be temporary only.

Source: ReptileCare.com