I don't have a screenshot of any Pikachu in my possession, but I just evolved my latest one into a Raichu.

Note the lightning bolt on its tail.
Male Raichu have larger/longer tips than females, as seen here:

(left: male, right: female)
Clearly the one I have in Pokémon GO is a female based on the model used. But the Pikachu I had did not have a heart in its tail. You'll just have to take my word for it.
In fact, I've never seen a female Pikachu model in Pokémon GO. If you have, that would be very interesting. Better yet, if you have a Raichu, look at its tail for me. Male or female model?
It seems like the development team just grabbed the female Raichu model and ran with that.