You don't tell her, you ASK!
One good way is to ask her if you can go down on her. And you can tell her what you like.
she doesn't have a penis
If you have a penis your probably is a boy.
Hose and you tell a Girl to put her mouth on the end of the hose and when her mouth is on the hose you sticky tape around it and then when you are finished take a photo of the girl with a hose in her mouth and then you turn on the hose and all the water will pour into her body and she will be a Water Balloon.
The male has a penis and the female doesn't.
A boy has a penis and the female doesn't.
if it is seen by a girl and given the OK
the male has a penis and the female does not
You can tell a boy horse from a girl horse by checking it's genitals. Males have a penis, Females have a vagina.
the boy has a penis and a girl has a vagina
You can see the penis on a male just like on a dog
If it uses a penis to pee then it is a boy.
Does your dog have a penis? If yes, then he's a male. If there's no penis, it's broken. Return it for a new dog.