A female Pikachu has a v-shaped "dent" at the end of its tail. Male Pikachu do not have this dent. However, both genders' tails resemble a lightning bolt, fitting their Electric typing. In Black and White, male Pikachu have less black covering their ear, this is only shown in frontal sprites. Also female Pikachu have slightly longer ears, this is also only shown in frontal sprites.

But for Ash's Pikachu:

In Where No Togepi Has Gone Before! it was confirmed that Pikachu is male. Voice actress Ikue Ohtani stated prior to this episode that the writers had intended to keep Pikachu's gender ambiguous so the fanbase can connect with him more easily.