No, since the gas is a liquid in the cylinder, dispensed as a gas.
The only way to know what is in the tank is to weigh it. The tare weight is stamped on the cylinder, but of course if you've got a regulator and hoses on it it won't be accurate.
It seems like a co2 tank at room temperature will stay at like 800 psi forever, then drop to 0 when it's empty. One in the fridge will stay at 500 psi, until it's almost completely empty, then drop to 0 in a few minutes.
Broken Leg Brewery
Giving beer a leg to stand on since 2006