How are leaks in door rubber seals detected? There are few reported incident where the rubber seal on the door was damaged and it took numerous hours to find the cause and then find the location of the damage. I want to know the ways to detect the leaks mostly for the big passenger jets.

I am not sure if this can be detected during maintenance checks or even aircraft is in air.
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