Set Appliance State:

Mains Power Off
Set Multimeter to Resistance Set meter to measure resistance
CH Demand
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Reset appliance as necessary and allow it to run until its shuts down with overheat fault code.
Isolate Mains Power to appliance as per TOP's P4
Disconnect wires from the overheat stat
Measure resistance across the terminals of the overheat stat.
Expected resistance short circuit (zero resistance).