Keep tapping the side of the tank with your knuckles. Notice the sound. Tap the other tank (full), notice the sound. When you know one tank is finally empty (because there no flame at the stove or the furnace is heating), tap both again. You'll hear the difference in the tone. Remember that tone. When both tanks are full again, they'll sound almost the same. As you use one tank the tapping pitch will begin to change. After a while, by simply tapping the side of the tanks, you'll be able to tell how full they are. I've been doing this for years.