You don't want to run out of propane halfway through your outdoor dinner party. Culinary weblog Chow shares a quick way to estimate how much propane is left in your tank; disconnect the tank from your grill and pour a glass of warm water along one side of the tank—areas with with propane will absorb the heat from the water and feel cold to the touch and areas without propane will feel warm.
This isn't a method for exact measurement, but knowing that a full tank provides around 20 hours of grill time should let you estimate if you have enough propane for your next grilling party.
How to Tell How Much Propane Is Left for Your Gas Grill | Chow