Seeing the "boys" is the best way to tell, definitively. From just a cursory glance at the photo, the length of their legs, their girth, shoulders, head can all indicate a male vs. female. Usually, males are bigger than females. That can be dependent on their age, food availability, etc. Fur can camouflage their true identity and often makes them look bigger than they really are. I have seen some extravagant claims of weight based on the fur. Scales tell the tale.