How do you test the amount of carbon dioxide in carbonated beverages?

Technically, since the carbon dioxide does have weight, if you could somehow procure an unopened can/bottle of soda that is somehow flat, and another can/bottle of soda that is carbonated and took the masses of the two, the amount of CO2 would be the difference in the weight. The difference in the weights of the cans is pretty much negligible if you do this for enough trials.