I used to live in a big city and walk in a crowded downtown every day and it became sort of a game to me to try to make assumptions about people from behind before I caught up to them. NO BREASTS. NO MAKEUP. NO VOICE to be making assumptions off of.

Things I found out:

You can tell someone is male by the way the walk even when they're in heels because of their calf shape: men have less body fat, so the muscles are more defined, giving an a more angular look, especially on the interior silhouette.

If they're wearing pants and a coat, you can tell someone is male by the way they walk: when they take a step forward their knee extends and their foot ends up closer to the center of their body. When women step forward their knees tend to angle inwards while their ankles kick outwards more, because of hip joint placement).

Even men with long hair can be pegged as men from the back, usually due to the skull shape, and if they have a ponytail, from the size of the neck in proportion.

Additionally, male shoulders tend to move more when they walk (right foot forward left shoulder forward) whereas women tend to be more balanced above the waist, possible due to a lower center of gravity.

But I've also had ftm classmates who I talked to for months before finding out they were trans.