They saw her face.
Hail to the king, baby.

Is that what you would have done?

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She looked like a girl to me. Her facial features are female. Her cheekbones, chin, eyes, nose all typically female. You shouldn't automatically associate short hair with being male.

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pre puberty that's true as male and female bodies are almost identical, including bone density, breast tissue and testosterone levels.
But by 9 or 10 the difference becomes more than social identifiers (clothes, hair and so on) because body shapes change.

At a very young age, perhaps not, but by 11 or 12 years old, the distinction starts becoming apparent.

I don't think that applies to kids.

Up close and in person, you can usually tell. Male and female skull structure have significant differences that most people will pick up on, even if they're not sure what they're looking for. One of the strongest indicators is that an average male skull has a more prominent eyebrow ridge than a female skull.