Gender Identity is not human nature, but rather the product of culture and language association. There have been plenty of cultures in recorded history with multiple - what we would call today - Gender Identities.
Language and its impact on perception of reality is a very interesting topic of study.
That's not what I meant. Our brains are programmed to make as complete an image of a person as possible when first meeting them, including all attributes that can be inferred from their exterior.
Oh, easily fixed: If you aren't able to tell if someone identifies as Masculine or Feminine; there's a solid chance that they're Non-Binary. If you aren't sure though, its always safe to refer to and think of someone as "They/Them" and Non-conforming to traditional gender roles.