Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove tubes from APS noting their connections

Measurement:

Measure between the two terminals on the air pressure switch.
Resistance should be open circuit.
Note:
Test shows APS removed for clarity but it should be
possible to do with APS in situ unless access is poor.