Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect X20 from PCB

Measurement:

Test 1:
Measure resistance across connections 9 (blue) and 18 (red) of disconnected connector X20.
Expected resistance of approx 52 ohms.
Test 2:
Measure resistance across connection 9 (blue) of disconnected connector X20 to a suitable earth point.
Expected resistance should be greater than 1 Mohm.