Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Turn Hot Water Tap On
Disconnect the two black leads from the pink leads at the inline connectors on the flying lead from the DHW flow switch

Measurement:

Test 1:
With hot tap fully open measure the resistance between the two disconnected black leads.
Resistance should read short circuit.
Test 2:
Turn off the DHW tap and repeat the test.
Resistance should read open circuit.