Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
DHW Demand
Disconnect the two pink leads on the DHW flow switch.

Measurement:

With hot tap fully open measure the resistance between the two spades resistance should read short circuit.
Turn off the DHW tap and repeat the test, resistance should read O.L open circuit.