Remember, the bladder fills with water, and the air fills the rest of the tank. It's quite possible to have air leaks with an intact bladder.
Okay, can one check the air pressure 'without' draining the tank? And if so, what should the reading be?
If I check it right now the air pressure will read the same as the water pressure, and that doesn't sound right to me.
The back tires on my pickup will read the same psi whether the bed is empty, or has 1000 lbs in it, so I wouldn't expect the water pressure in the bladder to effect the air pressure.