Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove change-over valve microswitch cover
Remove plug from base of microswitch housing

Measurement:

HW tap OFF:
Resistance between black and blue wires open circuit.
HW tap ON:
Resistance black and blue wires short circuit.
Replace plug and cover after testing.