Any type of pattern matching the brain does is complex and contains subconscious ("non-explicit") heuristics. Right now I'm reading a book[1] in which the author asserts that paramedics sometimes "just know" that somebody who they meet in some non-medical context is about to have a heart attack. Turns out there is a characteristic change in blood flow in the face that announces a heart attack, which an experienced paramedic can unknowingly pick up on!

Some candidates for non-obvious clues are:

• The amount of contrast in somebodies face. The more contrast, the more feminine. [0]

• Proportions that depend on height. Even when you can't tell how tall somebody (because their body is out of frame), your brain could pick up on the jawline-neck ratio.

• Skull shape. Men's eyes tend to be a bit more deeply set, i.e. they have a thicker browbone.

• Posture and gestures. Somebody who tries to perform gestures they are not used to may unintentionally exaggerate.

• Women's hair is much thicker than men's. As a result, identical haircuts end up looking slightly different. Women's long haircuts naturally have more volume than men's.

• Skin texture is influenced both by sex hormones and chromosomes directly.

Hope this is helpful!

[0] https://www.researchgate.net/publication/26883022_A_sex_difference_in_facial_contrast_and_its_exaggeration_by_cosmetics [1] Atomic Habits (Pop sci about habits, opinion so far: generic self help thingie)