I recently took these images of a female and a male Giraffe on Thanda. Without looking at the rest of the animals it is quite clear which one is the male and which is the female.
Smooth lines and hairs on top of the ossicones (the horn-like extensions of the skull) define the female head. A rough outline with many calluses from fighting other males and being bold on top of the ossicones shows a distinctly male head.
So, when looking trough binoculars and seeing only the head of a Giraffe sticking out from the trees, it is quite easy to tell what gender animal it is!