Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Gain access to the mains connector on the connection PCB
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Using either a socket and see or multimeter re confirm the
polarity on the incoming mains supply to the appliance.
Is the polarity correct ?