Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector X4 from the PCB

Measurement:

Test 1:
Measure the resistance between pins 1 and 2 on connector X4.
Expect a resistance greater than 1Mohm.
Test 2:
Measure the resistance between pin 1 and the earth at the incoming mains supply.
Expect a resistance greater than 1Mohm.