What you could do, it might be too late with the current tank, but weight it empty. Then when you get a new one, weight it and see how much it weighs. You might be able to find the weight of the tank somewhere on the tank it's self.
Another trick, get some really really hot water, about 1/4 gallon to 1/2 gallon and pour it down the side of the tank. Feel the side of the tank. You should feel a line between hot and cold on the side. THat's how much propane is in the tank.
Kegs: Galaxy Pale Ale, Chocolate Oatmeal Stout
Primary: Berliner Weiss v. 1, Berliner Weiss v. 2, pilsner