I jack a tire slightly off the ground, pull the emergency cable, rotate the tire to see if it locks, put the emergency cable back in, lower the tire, and repeat with the other 3. After verifying all 4, I hook up to the truck, start rolling 4-5 mph, and manually activate the brake controller to make sure the truck controller is working correctly. Gravel at the storage facility so when I activate the controller to max the tires easily skid. When I have a chance, I do the same on pavement to make sure of the stopping power.
When I noticed my brakes getting weak like that, I adjusted the brakes (manual adjustment) and there was a major difference in stopping power.