Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the green leads from the DHW flow priority pressure switch

Measurement:

Test 1:
Measure the resistance across the microswitch.
Expect open circuit.
Test 2:
Turn hot tap ON.
Measure the resistance across the microswitch.
Expect continuity / zero resistance.