checking disc brakes

Milwaukee has a battery powered borescope...
Plumbers use them too.

 

Just stick your hand in by the pads, you can feel the thickness of the pad. If they have dust covers, just take them off so you can see the wheel seals.

 

Looking for rust on the disc to make sure they are working properly is about all you check. Could look for cracks plus checking the pads. Probably have to pull tire off to check them

 

As long as the rotors are shiny I check my steer axle when I put tires on and I check my drives once a year when I rotate them.

 

If I had one here in the shop I'd take a picture of my hand there. You feel with the tip of your finger.
Been able to do it on FL/Volvo/Pete & all my trailers.

 

How many miles on truck or brakes? You'll be amazed at how long they last, local or OTR.

I pull a wheel personally but I know that's not easy for some. You could drop by a tire shop and ask them to pull a wheel for you to check them out. Probably just cost you a few bucks since they aren't breaking down a tire.