If i'm correct you can drop a gas bottle in water to measure it. The more full it is the deeper it will float into the water. If it's really full, it will sink.
Not sure if the above is correct.
It is also possible to check with putting pouring hot water down the side of the gas bottle to heat the steel. The point at which you feel the temperature change is the level of the gas inside the bottle. But that would be a bit complex for a game.