You can only really sex properly a gecko over 6 months unless your very good at it !
Hold the gecko firmly and look at the underside, just where the body meets the tail.
You will see the vent.
On males, just above the vent, between the rear legs, you will see what looks like a small arrow head made up of dark brown spots. These are infact pores that secret a waxy type liquid. Just below the vent, on the tail side, you will see two distinct 'bumps', often refered to as his b**ls. In fact these are his hemipeinal sacks, or inverted sex organs.
On femals, you may just make out a very faint row of pores, but the are often so small you cant see them. this is because the females do not secret the waxy substance. Aslo, just below the vent, they have a faint single bulge in the middle of the tail, but again often its so faint you cant make it out.
There is a great photo around somewhere, but cant lay my hands on it right now, but the above should help