How do you know if an African fat tailed gecko is male or female?

Ask your vet/someone at the pet store. If you don't know what you are looking at, it's very hard to tell the gender.

You won't know until is either grown or about to be grown, but with males you see dots under it going into a shape of a v like this below

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but females don't.

Above the tail on their stomach there's a v shape that will be outlined will black centered scales.

A male Leopard Gecko has a set of chevron shaped pores on its underside and a bulky tail base.

the male has a distinct row of preanal pores in a v-shape just above the vent and also has 2 bulges just under the vent while the female has neither

Males have a V of darker dots in their pelvic region. Females do not have this. Also, males have 2 large bumps at the base of their tail which is absent in females.

If you are having trouble picking up the gecko, put it in a see through plastic container and look on the bottom...

The male will have a V on hes under belly at the two back legs. The female will have the V but you can't see it as well