boys grow pubic hair on their crotch.

Pubic hair is naturally found in the frontal genital area, the crotch, and sometimes at the top of the inside of the legs; these areas form the pubic region.

Pubic hair is not found inside the Vagina.

From Wikipedia: "Pubic hair is hair in the frontal genital area, the crotch, and sometimes at the top of the inside of the legs; these areas form the pubic region." In other words: on and around her vulva.

By looking.

I normally have a rather thick patch of dark brown pubic hair, about the same color as the hair on my head. However, in early March, I trimmed and shaved the crotch area, and the hair is slowly growing back.