Well, I've never seen a bear with two breasts. That's pretty funny.
The shape of the head that's already been mentioned is a good indication.
When a bear is on all fours if the hind quarters are slightly higher than the shoulders it's a sow.
If the shoulders are higher than the hind quarters it's a boar.
Like Elkcrazy8 said if you look at enough you will be able to tell.
It takes looking at a lot of bears to know for sure.