Along the same lines, propane companies have started to fill standard 20-pound tanks with less fuel (15lbs) while charging the same price as before (when they filled the tanks 12% fuller)

As far as I can tell, you won't find this information anywhere on the tanks themselves.

Here's the story from Consumer Reports: http://blogs.consumerreports.org/home/2009/05/amerigas-blue-rhino-propane-gas-tank-exchange-gas-grills-home-depot.html