They can tell because the CAD models used for Male and Female skull creation are notably different.

The male skull is noticeably flatter at the front because cavemen tended to punch each other in the face a lot

The fact it's doing the goddamn DreamWorks face is the cherry on this

In the physical anthropology world we'd use terms 'grassile' and 'broad' to avoid gendering a piece of bone. Unless there'a fleshy matter or a baby skeleton wedged in the pelvis, (and even then...) you're not going to be absolutely sure.

You can tell the diagram is fake because the female skull doesn't have a pink bow permanently attached to it

a fleshy matter or a baby skeleton wedged in the pelvis

Sweet Lord Jesus

The fossil record hears ye. The fossil record don't care.

I hear that's one of the reasons for facial hair too

The whole basis of phrenology is just the names of the CAD files.

males-whites-ubermensch.stp

female-.stp

They never stopped to check if the files are actually different aside from the name though...