Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove rubber boot from the system pressure switch
Disconnect black wire from terminal marked C (common) (1)

Measurement:

Measure resistance across the N/O (normally open) terminal (blue wire), and the C (common) terminal, where the disconnected black wire was connected.
Resistance should be short circuit.