Above it correct, but it makes no difference if its disc or drum.
tow it and stop, then get out and put your fingers on the drum or rotor. If it's boiling hot, then they are working.
If it has disc brakes, the easiest way would be to hook it up to your truck, but don't connect the wiring (in case it has a reverse solenoid). Then, try backing up in reverse. The pressure from backing up should actuate the brakes and the trailer should be very difficult (or near impossible) to back up.
The reverse solenoid is there to eliminate braking when in reverse. It is energized by the back-up light circuit. So the only way to confirm the brakes work in reverse is to unplug the harness for test. Driving in reverse is kind of like stopping while going forward, the master cylinder is "surged" upon . . .So they dont need power to actuate the solenoid?