Your best bet is to weigh the tank. Somewhere stamped on the tank is the tare weight (the weight of the empty tank), may be marked as "TW" or something to that effect. Weigh the tank, subtract the tare weight, and that's how much gas you have left. Granted, what you need to weigh is the tank only, no regulator or anything attached. If you take off the reg, make sure you put it back on properly and leak free. As long as there is liquid CO2 left in the tank, the high pressure gauge isn't much use.
Granted, I guess it could tell you the temp without a thermometer...