Is the anyway or any device that can tell you how much propane is in an 100lb tank?

You can pour hot water over it adn you will see once it settles there will be beads of sweat along the start/stop part of the propane..propane being cold and the hot water poured over it builds condesation on the tank itself..where the beads of water start to form..is also the level of the propane in the tank