Mix up some dish soap in a windex type bottle with water to get a soapy solution.
Jump wire the compressor to force it to run. Once you do so, spray the lines and the air bags and see if bubbles form. If so, you know you have a leak.
There is the switch as mentioned above, by forcing the compressor you bypass the switch to some degree. You bypass the on / off aspect of the switch but NOT the solenoids that allow air to get into the actual bags of the shocks.
So the bubble test is only good up to the shocks UNLESS you can unhook the swing arm of the level switch and move it around to get the solenoids to open up to allow air into the shocks.
If all else fails, get a replacement air shock line kit and hook it up to the factory shocks and mount the shrader valve somewhere you can get to it with a compressor and operate the shocks manually. This will bypass the rear load / level switch as well as the solenoids at the shock locations.
If the OEM compressor still works, you can hook it up to a switch and actually use it as the air source with some line adapting...
I use to work on MANY MANY Lincolns and they had the same set up.
The diagnostic system is bizarre. For Lincoln, key on and off 8 times, foot on the brake, then open and close the drivers door two times. This enters the test mode. Then each time you open and close the door it goes to the next shock.. Good times.. LOL