Hold a compass down close to trailer brake drum ----lower area.....then have someone pull pin on emergency brake switch.
Does compass needle deflect towards drum
NO....then magnet is not being energized so wiring needs to be checked
YES...then magnet is being energized and brakes need adujstment
Yup - great idea BUT...... have you tried finding a magnetic compass these days? I tried several major stores last year with no luck.
The other way is to put your ear near each wheel, if the brake is working you will hear a humming sound and if you touch the wheel you may feel a vibration.