One thing I neglected to mention over the course of this conversation: IR temp gauges.
I make a habit of checking the trailer hub temperatures at each stop (usually for tow vehicle fuel). When properly adjusted, they seem to run in the 97 to 110 degree F range. Any increase means that something is amiss, causing friction, baking the grease, and most likely, signaling imminent bearing failure.
The device I use is from Radio Shack, but there are a ton of them out there:
Simply point the device at any exposed portion of the hub, press and release, and read the temp. Pretty simple, and takes about the same amount of time as walking around the trailer.
Interestingly, the tow vehicle hubs tend to run nearly 100 F warmer than the trailer hubs, I figure it's probably due to the disk brakes.