The only way to tell if a propane tank is empty is to weigh it. This can also tell you if there is any propane left. The typical tank for a gas grill can hold 20 lbs. of propane. There is also the weight of the tank to consider. The tare weight (TW) noted on the tank is the weight of the tank when empty. Once you know the TW, the amount of propane the tank holds when full and its current weight, you can figure out how much propane, if any, is left.
Weigh the propane tank.
Look on the tank for the tare weight. Typical TW for a 20-lb. tank is 18 lbs.
Subtract the TW from the combined weight of the tank and the propane. This will tell you if your tank is empty or, if not, how much propane is left. For example, if you have a 20-lb. tank, the TW is 18 lbs. and the combined weight is 23 lbs., you have 5 lbs. of propane left, or 1/4 tank.