Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove both wires from the limit thermostat

Measurement:

Measure the resistance between the terminals of the limit thermostat.
Expect continuity / zero resistance.
Is the resistance measured as expected?
Note:
Image shows the limit thermostat removed for clarity but the test is carried out with the overheat stat in situ attached to the primary heat exchanger.