Since we started getting trucks with disc brakes, most of us (who care) questioned the lack of guidance on how to actually inspect them.
Well, here is everything you need to know. Plus some. Too much? You be the judge.
1st: Just like all brakes look for signs of a leaking wheel seal, that could contaminate the brakes.
Pad thickness:
Out of service thickness is anything less than 1/16 inch. FedEx replaces at 1/8 inch or sooner.
The rotor: It has a "chamfer" (45 degree bevel) around the edge. When that bevel is gone, so is the rotor. Obviously scoring or signs of metal on metal contact would be a deadline item.
Wear indicators: The pads have a groove that should be visible, but there are indicators on both the carrier and the caliper that (when they meet) indicate the need for a wheel off inspection. Likely time for pad replacement, once in there.
The above is very difficult to see on the Drive axles, unless you lay on the ground with a flashlight and look. The steer axles are very easy to see under the hood, if you look, again with a flashlight.
The following video is very good, even showing how easy the pads are to replace.
And this one shows things from a roadside DOT inspection point of view.
