Get a gauge. There are all sorts of devices available. There are inexpensive paste on strips that will display the temperature change at the the level of your liquid propane. There are expensive hand held ultrasonic tools that can detect where the liquid or gas is inside the tank. And in the mid-range, you will find mechanical or electronic tank scales. These may be built-in or separate from the grill and can be calibrated to tell you how much propane you have left with an analog or digital display.
How to Tell How Much Propane is Left
Empty propane tanks are heavy, so it can be tough to figure out simply by weight if you have enough propane to last a through your cookout. Try some of these tips to see how much propane you have left in your tank.
Tips for Your Propane Tank
Weigh it. Most propane tanks weigh around 18 1/2 pounds when they are empty. Place your tank on a household scale and subtract 18 1/2 from that number. From there, you can determine how much you have left. An average size (500 sq-in) grill will use about 1 1/2 pounds of propane an hour cooking medium to high.
Pour water on it. Turn off the valve on your tank and disconnect it from your grill. With a small bucket of warm water, pour the water from top to bottom on the side of the tank. Then feel the tank. The tank will feel cooler where there is propane and warmer where there isn't.