Adult male crested geckos are quite easy to spot as they will develop a very large hemipenal bulge just behind their vent. This bulge often will appear when the male gecko is between 5 to 9 months. Sometimes females will develop a fatty layer that can appear as a bulge and be mistaken for a male.
The pores in a male can usually be seen with the naked eye in adult crested geckos. These pores allow the gecko to excrete a waxy substance containing pheromones. These pores can typically be seen above the vent appearing as small dots in a bit of an upside down v shape.
In young crested geckos it can be very difficult to tell whether or not your gecko is male or female, for these animals a magnifying glass can be used to get a good enough look to see a line of pre anal pores spreading partially up the leg. Females will not visibly have these pores.
There are other tells that sometimes breeders will use, such as males typically have larger heads than females, thicker bodies than females, and larger spurs than females, but these don’t always pan out. Sometimes females will have larger heads or larger spurs. So this method is not always a guarantee.