Grizzly skulls have more overhang at the top/back of the skull. If you stand them up on the back of the skull (where vertebrae would start), my grizzly skull wants to fall forward onto the teeth (just the skull without lower jaw). My biggest black bear skull stands up almost vertical. I also checked and the brown bear skull is like the grizzly and falls forward if you stand it up. I don't know if all follow this pattern but my limited samples do.
The grizzly skull has a way more pronounced, longer and taller ridge bone along the top of the skull starting a couple inches behind the eye sockets. Black bears have one, but the grizzly's is significantly deeper.
Canine tooth spacing on a decently large boar black bear is about 2-1/8" center-to-center. On a decent interior mountain grizzly it is about 2-1/4" so little difference there on my samples.
Also, as measured by B&C and P&Y, I'd expect that any bear over about 20" has a chance of being a grizzly, over 21" likely a grizzly, and over 22" almost certainly a grizzly for MT. Google P&Y scoring forms for bears and you'll see how it is measured if you aren't familiar with the system.