To check the gas level in your propane tank, take a cup of steaming water and pour it over the side of your tank. Next, feel the tank from top to bottom, and look for the point where the tank changed from feeling hot to feeling cool. This is the gas level.
You can also determine the amount of gas by looking for the weight. The tare weight, or empty weight, is usually stamped on the collar of the tank. Since the standard propane tank is normally filled with 20 pounds of gas, you can easily determine the amount of remaining gas by weighing your tank then subtracting this weight from the empty weight. A normal-size tank with a 20 pound capacity weighs about 17 to 18 lbs. empty and about 37 to 38 pounds when full of propane. A full-size, 35,000 Btu gas grill will cook for 30 minutes per pound of propane.
According to the Eastern Gas Company, an above ground propane tank is considered full at 80 percent of its capacity. This is to minimize risk of explosion by leaving room for propane to expand when the temperature changes. Some propane gas grills are made with handy speedometer-like calibrated gauges that show the percentage of gas remaining in the tank.