Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance

Measurement:

Test 1.
Measure for resistance across the two wires on X21.
Expected resistance approximately 4.5kΩ.
Test 2.
Measure for resistance across one of the wires on X21 to a suitable earth.
Expected resistance greater than 1MΩ (or O.L)