I never went that radical, actually cutting a plug open, lol. I just look down into them as best I can with a little flashlight. It's pretty easy to see the "smoke" ring if it gets too high, like in your one pic.
One time I was experimenting on my '78. After all, that's what carb fine tuning is all about really, experimenting, lol. The plugs looked good with a "smoke" ring down near the bottom of the porcelain, but I wanted to see just what difference moving the needles a step richer would have. Would the bike run better? Would it perform better? Would the plugs turn black as a moonless night, lol? Well, it didn't seem to affect running or performance much, and it didn't turn the plugs all black, but the smoke ring did climb up so it was now covering 2/3 to 3/4 of the porcelain. Definitely too rich. Needle position has a big effect on plug color because it's tuning the midrange and that's where you spend most of your time running.