Black bears are the hardest big game animal to judge!
Big bears will have a noticeable seam of muscle in the middle of their head. Their ears will appear small, wide spaced.
Their chest, shoulders and neck will be thick. Big gut etc...
The toughest ones are those bears that are between 6' to 6-1/2'.
When you're trying to judge the size of any bear, get as close as possible. The closer you are the better you can gauge the size of the bear. I like to get right in on them, like 20-30 yards. The closer the better.
The really big bears will just be obvious, even at long range. But I've been fooled by bears, that looked good until I stalked close to them, then realize they were small.
References that help a lot are track size. If you're baiting, you should regularly rake the ground on trails, so that tracks can be distinctly seen, to measure the width of the track. That helps a lot; rule of thumb is to measure the width and add 1. A 5" wide track will be made by a 6' bear; a 6" wide track is from a bear that will be 7' roughly.
Another thing is how big the bear measures up something of known size, like against a 45 gal bait barrel. Trail cameras might help capture pics of your bear(s) hitting your baits that you can compare their size relative to your bait barrel.
Here's a big mature boar. See the seam in his head? Ears are wide, relatively small etc... think, heavy body.
Edit to add:
Another thing I do is I watch every bear hunting video I can. Jim Shockey has some great bear videos.
Cave Bears on the Pacific Rim is really good.
But there's lots of others that are good. I bought one recently made in Alberta,
Living For the Rut, Bears III. In that video they kill 2 B&C bears. Monster bears.