How are leaks in door rubber seals detected?
Testing the doors (and windows) by pressurizing the cabin eats into the aircraft's pressurization cycles. Instead vacuum is used to simulate the in-flight conditions.



A special fabric and a sheet with attachments to hoses are attached to the outside where the door/windows are to be tested. Vacuum is created between the fabric and fuselage, thereby locally simulating an in-flight differential pressure. If the vacuum doesn't hold, it means there is a leak, which can be pinpointed from the inside with sensitive equipment.



Image and reference: Lufthansa Technik AG