Big Stick from AK used to claim how he could nail skull estimates on a live bear. I'd guess if you knew the average top end bear's typical skull size for where you're at, you could be close by just matching a body size of a boar to a skull size. I'm not buying there's a real way to estimate a skull length/width on a bear's melon when covered with a few inches of meat and hide.
Bear judging is pretty tough, I'd say the best advice I could give is to look at them as long as you can, get as close as you can, and compare them to things around them of known or assumed sizes. I can think of several times in the year past misjudging a bear, where I wish I would have noticed how they compared to nearby stumps, bushes, sage, etc. Pay attention to their shape and how they walk, etc. Mostly the times you get into trouble is when you shoot too quickly, just my thoughts. I am always interested in how other people can tell if they are big.
The bear walking in the snow on the video above was a good one for SW MT, 6'11" square and 18-7/8 skull, 10 years old. I could tell it was big and lanky long by watching it walk down hill through all that snow. I've seen some short squatty furred up sows that at first glance appear to be a lot bigger, that were actually 1/3 the size of that one in the video i posted.