[ciiiicii]

Facial structure, body shape, voice. Humans have sufficient sexual dimorphism and are attuned to the differences enough to make this distinction correctly almost all of the time.

This is how we can, in most cases, ascertain a person's sex regardless of how they're attired.

It's also why the other commenter's view of women being "clothes, hair and makeup" is so absurd. A change of clothes, a haircut, and wiping off makeup doesn't somehow change women to men, or remove the ability of others to recognize sex.

Also, even if a person manages to disguise their sex or impersonate the opposite sex, this doesn't change the reality of their sex. Just may obscure it from observation, for some observers.

[novemp]

Then you have, in fact, mistakenly gendered trans people correctly. Just wanted to check, so thanks for letting me know.

[ciiiicii]

A friend of mine has a cat, which she's had since a kitten. When the cat was given to her, she was told it is male. However, when she took the cat to the vet for vaccinations, she was informed that the cat is in fact female.

A mistaken assumption doesn't change the underlying reality.

If she hadn't taken the cat to the vet and hadn't otherwise realized that the cat is female, and later, the cat had become pregnant, then she wouldn't be exclaiming how amazing it is to see a male pregnancy. No, she would understand that the cat is female, because only female cats can be pregnant.