There are a couple of ways:
1) Check the switch over, depending on it's style- the dot in the middle will be orange when the tank it is pointing at is empty. It'll be puling from the other tank, but that gives you time to replace the first one. Here's mine:
2) Weigh the tank and do the math. A normal-size tank with a 20-lb. capacity weighs about 17 to 18 lbs. empty and about 37 to 38 lbs. when full of propane.
3) Turn on a stove or something that is drawing propane and then pour a cup of scalding hot water down the side of the propane tank. You'll see a frost line show up showing the propane level.