look at the gears for gouging or flaking.
you can also check the backlash and record it. then later on you can check it again to see if it is getting more. this would tell you if the gears and/or the bearings are wearing out.
I think most of the time the bearings are going to get loose and then start causing excess wear to the gears. the gears are normally pretty stout if the fluid is kept good.
Also, every once in a while, crawl under the truck and shake the rear u-joint like you are checking it but pay attention to the pinion yoke also. if the yoke is moving, the pinion bearings have worn and are now loose which is another indicator. gear noise is also another clue that things are getting loose in a diff.