Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the wires from the air pressure switch (noting their locations)

Measurement:

Measure the resistance between the common (3) and normally closed (1) terminals.
Continuity / zero resistance should be measured.
Note:
Terminals are numbered on the air pressure switch.