Keep in mind caramelization is a bad term to use in conjunction with wort. You cannot get actually caramelization until the water is gone and the temp is much higher than boiling.
Correct, but this is actually valid in the context that it's being used here. When you are doing a side boil of first runnings for a Scottish ale the point is to boil it down to caramel. I don't use a candy thermometer, but I go until the wort looks and tastes like caramel. And it solidifies when you drizzle it on ice cream
I took a few pictures when I was brewing my Wee Heavy yesterday since there have been a few threads on this subject recently. I'm not sure how they came out, but if there's anything worth posting I'll try to share them later.
Basically, the biggest sign that it's ready (to me) is taste. It's tough to know until you've done it, because the flavor of the concentrated wort is already a bit toffee-like up until you cross over into the caramel zone. But the flavor is strongly and distinctly caramel-like once you get to the right point, and noticeably different from the pre-caramelized wort.
I've only done this a couple of times, and I've never made candy. I don't know if I trust my eyes more than my palate at this point, but I did notice one visual indicator that seems to coincide with the caramelization. The bubbles changed from rising evenly to rising in mounds. Instead of what seemed more or less like a sheet of regular-shaped bubbles rising out of the pot, one side of the pot tended to mound up a bit more than the other, and the bubbles in the center seemed smaller than the ones around it.