Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect leads from the DHW sensor

Measurement:

Measure resistance across the exposed terminals.
The resistance will depend upon the temperature of the DHW and should lie between 500 Ohms and 1500 Ohms.
The cooler the water the lower the resistance.
For the boiler to operate at full gas rate the resistance should be between 500 Ohms and 1000 Ohms.