they have magnetic gauges you can put on the side of the tank..I have never used one before so I can't say how good they work...or feel the side of the tank, propane is colder.... ...got this info below off the internet Safest way to do it is to take the tare weight or water weight of the tank. Typical tank is about 4.5 lbs but the actual number is stamped on the protective neck around the valve, it is T.W. then what ever number. Step on a home scale and weigh yourself record this number. Pick up the propane tank and reweigh yourself. Subtract these two numbers then the T.W number, The weight left over is the propane weight. Only reason you should weigh the tank with your weight is that most home scales are less acurate at lower numbers. The propane weight divided by 4.5 will give you roughly your gallons of propane you have left. The typical grill tank holds about 4.2 gallon at 80% capacity which is the max you can fill them by law. Easiest way to tell is to crack the bleeder valve and judge the amoount of propane in it by the amount coming out of the bleeder valve combined with the weight of the tank. DO NOT DO THIS IF YOU DON"T KNOW WHAT YOU ARE DOING

I have used the magnetic guages that songofjoy speaks of. I think this the best way to keep track of propane. You can buy them from Harriet Carter, Lillian Wright and other home gadgett catalogs.

In the morning go out and look at the moisture level on the outside of the tank.