sorry I thought you ment the solenoid plug not the pump plug
if you have a multi meter you could put it across the solenoid and see if it switches
This is just a thought but if you let the motor cool down then either pump the brakes without engine on or open a bleed nipple at the rear and bleed it.
This should drop the pressure so when you turn the ignition on the pump will come on it will biuld pressure and the solenoid should change state this would prove the switch.
paul