Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Ensure DHW is cool (this can be achieved by running the DHW with the gas to the appliance turned off)
Disconnect plug from DHW temperature sensor (orange wires)

Measurement:

Measure resistance across the exposed terminals of the DHW sensor.
Note: Expected reading with water temperature at 18 degrees C would be approximately 13 kOhms