How can I find out how much toner is left in my printer, which is connected to an Airport Express via USB?
Attaching a printer using Airport does not provide the same capabilities as attaching through USB, and one of the things that will not work is querying the printer for paper/ink levels etc.

When you have the printer attached locally, the driver has the ability to talk over USB for lots of things, including these services, reading from a card reader that may be on the printer etc.  Once the printer is made wireless, these facilities are lost, and it effectively just becomes a dumb queue that you can drop a print into.  About the only feedback it can give is if the printer is offline or not.  If your printer is multifunction, you'll likewise lose the scanning/fax stuff also.

So unfortunately the only way is to hit up the printer direct, walk on over there and take a peek.