From the third series called Diamond & Pearl, the first female Pikachu "debuted" with the audience, the appearance almost identical to the male but the tail shape is different. The female has a half-hearted tail, unlike the male (Satoshi's) has a straight tip. The appearance of females in the series is not as dense as that of males. Not only the gender of Pikachu, the sex of other Pokémon has also been "hidden" by manufacturers for a long time. Two years after its launch (in 1999), the sex of other Pokémon was revealed.
Source: Uglyxmast
Source: Mumtee.com