On the left is an adult male Leopard Gecko. You can see that the bulge behind the vent is quite noticeable. That is where the hemipenes are located. You can also readily see the row of femoral pores shaped like a "V" in front of the vent.
On the right is a female Leopard Gecko. As you can see, she has neither the bulge nor a noticeable row of pores. There is a row there, but it is really not that noticeable.
You will have to wait til the Gecko is old enough, over 6 months at least to really be sure. Juvenile male Leopard Geckos look very much like females.