Is ash's Pikachu a girl or a boy?

It's easy to tell. A female pikachu has the little dent on the tail. The male dosen't. It's just like a ttly straight. And pikuchi doesn't have the dent so it's a male

^By the time Pikachu was introduced,there were no gender differences,therefore that´s not proof at all.

SPOILERS:

The proof that Pikachu is male is in "The Evil Togepi" episode,where a female Togepi uses Attract and he is affected.

Also,in the recent "Get Tsutarja with Attract?!",there is further proof,where Ash´s new Tsutarja,also confirmed female uses Attract on him yet again,with it working.