Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect one of the black wires from the safety thermostat located on the flow pipe, top right hand side of the appliance
Note.
Thermostat shown removed for illustration only

Measurement:

Measure resistance across the exposed terminals of the thermostat.
Expect continuity / zero resistance.