About the only thing you can do is to get an accurate scale to weigh your tanks empty. Note exactly how much each weighs.
Then just weigh them again later to see how much is left.
Your local field should be willing to help you with the weighing part if you don't have a scale.
Unfortunately, the field can't top off your tanks though, we have to dump them dry, then fill them per insurance regs.
i saw these strips for propane tanks (like for gas grills) and thought it would be a good idea for someone to concoct a strip like this for a co2 tank. maybe you could get a guage on your asa. know this wouldn't be the most accurate but if your tank got down to refill time you would know.
What? they dont make CO2 guages anymore? they use to have them on guns like the spyder shutter, they werent very accurate though because it was co2, you should know how many shots you get out of each fill though, like i get 1000 from my 20oz tank
As long as there is liquid Co2 in the tank, there will always be +-800psi in the tank. The only time a gauge will be useful is that point, that the last of the liquid boils off and there is still 800psi in the tank.
Counting your shots is the best way to determine how much Co2 is left to use.