You can only properly sex leos from about 6 months.
Males will have some markings on the uderside that females dont.
Look between the rear legs. Males will have what appears to be a arrow head made up of small round dots, pointing towards the head. These are in fact pores. They get darker as the male gets older. In addition, below the vent you will see two bulges at the base of the tail.
Females, whilst they do have these pores, have very much lighter and smaller pores, and often you cant see them at all. they also do not have the bulge at the base of the tails.