Although it can be difficult to know with certainty, there are clues in the shape and size of bony landmarks on the skull. You usually can’t sex other bones reliably, except for the pelvis.
With the skull, what scientists look for is the difference between what is called robust morphology, meaning typically heavier, larger shapes and sizes, and gracile morphology, meaning smaller and more “graceful” shapes.
Males tend to be more robust in the browridge, the mastoid process behind the ear and the lower jaw.
But with only one fossil example, researchers must rely on what they know of the species and make an educated guess. Often it can be unclear whether researchers are looking at a species that has a marked difference in size between the sexes, called sexual dimorphism, or an entirely different species.