Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the two wires from the low water pressure switch making a note of their locations

Measurement:

Measure the resistance between the two terminals where the wires were previously located.
Expect continuity / zero resistance.
Is the resistance measured as expected?
Note:
Test is carried out with pressure switch in situ and is only shown removed for clarity.