Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove the two wires from the low water pressure switch noting their locations
Ensure system pressure is correctly set

Measurement:

Measure the resistance between the two exposed terminal of the low water pressure switch where the wires were previously connected.
Expect continuity / zero resistance.
Is the resistance measured as expected?
Note:
Low pressure switch is shown removed for clarity but test is carried out with it in situ.