Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect wires from overheat thermostat
Image shows wires of flow thermistor disconnected
for clarity but this should not be required to carry out test

Measurement:

Measure the resistance across the overheat thermostat.
Resistance should be short circuit (zero resistance).