This gender determination cannot always be distinguished easily or with certainty while in the field, unless of course it is a female accompanied by cubs or the bear is in a position to expose its underside.
The females of the Black Bear species (sows) are typically smaller and leaner than the males. Their heads are more narrow with a flat looking forehead, plus their ears are generally bigger than a males ears. Female Black Bears are usually more shy and cautious when entering an area.
The males of the Black Bear species (boars) are typically bigger than the females. Adult males have more heavily muscled heads, necks, and shoulders than do females. The male head is wider and rounder than the females and the ears are generally smaller than a females ears. Male Black Bears are not as cautious when entering an area.
I personally enjoy whenever I get to see the adult boars, they are much more reclusive and nocturnal than sows, yearlings and young boars. They are the equivalent to big whitetail bucks as far as keeping a low profile (unless of course it is mating season). Most of the adult boars tend to have somewhat of an attitude the majority of the time, but that's one of the traits that makes them the superior beast of the mountains.
As with all things in nature, there are ALWAYS exceptions to every rule. Note the use of the word 'typically' above. The weight information is only useful for trying to make a quick determination in the field. I have personally seen several large sows that could mistakenly be identified as males (based upon the weight theory), which shows that hereditary traits can cause exceptions. The ears theory has exceptions also because nearly all juvennile bears seem to sport the 'big ears' for the first 3 years of their lives.
Another method of gender identification is the presence of nipples, most notably in the chest area (see above photo). Both male and female bears have nipples (as all mammals do), but the males have only two and the females have 6 functional nipples. With a female who has cubs, the chest nipples are much easier to detect. While they are lactating it is much easier to detect them as the area around the nipple will be swollen with milk. Another way to tell if a female bear has been actively nursing cubs is that the fur around the nipples will be slightly discolored from the saliva of the cubs (see below photo).
For more black bear photos, information and video clips, visit the Black Bears section of my Web site.