Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Gain access to the PCB
Disconnect connector X14 from the PCB

Measurement:

Test 1:
Measure the resistance across pins 4 (brown)
and 3 (blue) of the disconnected connector.
Resistance should be greater than 1M Ohm.
Test 2:
Measure the resistance across pin 4 (brown)
and a suitable earth point.
Resistance should be greater than 1M Ohm.