It can be hard to tell when you are about to run out of propane. This is why so many people will invest in keeping a spare tank on-hand to simply make a quick switch. If you only have one tank you might be able to get a rough estimate by picking the tank up and feeling the weight of it. You can also take a quart of hot water and slowly pour it down the side of the tank in a trickle. The condensation that forms will tell you roughly how much propane is left inside the tank.