If the brake controller is indicating that the brakes
are operating, a quick and dirty way to check whether the brakes
are engaging is to tow the trailer slowly and then brake a few times while a second person is looking at and listening to the trailer brakes
. Also, if you press the brake pedal while parked and have a second person standing next to each trailer wheel, the second person can actually hear the brake magnets engage/disengage. EDIT: I prefer to do this on a gravel surface.
Next, while parked with the engine off, you can jack up the trailer frame first on one side, then other side. With the trailer wheel off the ground, if you press the brake, a second person can turn the trailer wheel to check whether the brake is engaging. In addition, while the wheel is off the ground, you can remove wheel and inspect the brake (see youtube videos for how to do this).