After a dozen 1/6 barrel kegs I knew my co2 pressure was a little low. Normally the gauge was 500 psi. Before this last keg went in it was a little lower and after the keg was empty the co2 bottle was reading really low. However there was still gas in the bottle. The bottle is stamped with the empty weight on it and using a shipping scale I could verify it was nearly empty.
The markings on the bottle also are good for determining the manufacturing date. Every 5 years the bottle needs to pass a hydrostatic test to be recertified, so knowing when it was made comes in handy – especially if you are buying it second hand.
This info comes from here, they also made the bottle that I use for my kegerator.