'03 Buick Lesabre.

How can I check to see if the air suspension is leveling the rear end?

Occasionally I would hear the compressor running in the back. The other day I found a piece of rusted metal hanging down from above the muffler. When I crawled under the vehicle, I notice one of the three rubber hoses had deteriorated and fell off. One end of the hose was connected to plastic T fitting; the other end was just hanging there. I suspect the other end was attached to a nipple that was badly corroded, the lowest nipple from a little black box. I put all new hoses on the three way T fitting.

Simple .....turn your key to "on".....have you and another hefty guy, open the trunk and sit in it....after a slight delay, you should hear the compressor come on and pump up the air shocks.....when it stops get out......the rear end will be noticeably higher....after another delay you will hear a click, and the vent solenoid will open to let air out of the system, and the car should lower to normal height...

Those EVAP hoses almost always deteriorate like that, I am pretty sure those direct the air to the air dryer part of the compressor, which is supposed to make sure moisture stays out of the line, good job noticing that, and replacing them.
You may want to look over any and all soft EVAP hoses, there are a few under the hood as well, if I had a diagram showing where they all are I'd post it, but unfortunately I do not have one.