Note; if you have a very young gecko, it may be hard to tell the difference between male and female.
Geckos, like most reptiles, do not generally like being flipped over. The easiest way to check the cloaca is by placing them in a clear glass tank or on top of a glass table to observe from underneath. You may also want to try lifting them above your head but be mindful of sudden bursts of energy that may allow your gecko to slip out of your grip and fall.
Femoral Pores & Bulges
The two main things you will be looking for are pores and bulges. If you notice a bulge it is possible that the gecko is a male. However, it is not 100% definitive and may be difficult to determine if you do not have other geckos to compare. This makes the absence or presence of pores the easiest. Pores generally show earlier than bulges.
⦁ Males will have two bulges behind the cloaca and pores in front of the cloaca.
⦁ Females have no bulges or femoral pores.
See more in this video.