I hear they use methods such as you speak in the telephone industry with special listening devices.
We use a number of techniques. The most recent is the smoke machine. You hook it up to the system and it applies 1psi pressure with dense white smoke. The smoke rising from an area identifies the leak.
The way we have done door locks for over twenty years is all together different. We start by identifying the line to the reservoir. We disconnect it (taking the reservoir from the system). We put a gauge where the line to the reservoir was. The point here is to reduce the ammount of vacuum in the system so small leaks become more significant.
We then run the car in both the lock and unlock position and shut-off. We watch the gauge in both cases and no movement is the sign of a tight system. If the reservoir is in the system small leaks would take hours to show. Without the reservoir small leaks are usually apparant as visible needle movement.
Once the leak is verified as lock or unlock (basically the same thing on later single line systems). The side that is leaking is divided at strategic locations usually under the carpeting on passenger floor. The system tees there to the right doors, trunk, and gas flap. The individual lines are capped until the leak goes away. Once the culprit is identified the rest of the lines are hooked up. I never rehook a tested line untill all leaks are gone and then they get rehooked one at a time. The defective unit is identified then by operating the system and viewing which element isn't working. That will be the one that is plugged and was the leak.
On early two line systems both sides "lock" and "unlock" must be checked in this way.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician