Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove both orange wires from DHW demand switch

Measurement:

Hot Water tap OFF.
Resistance across outer terminals of switch = open circuit.
Hot water tap ON.
Resistance across outer terminals of switch = short circuit.
Note:
The centre terminal on switch is unused.