I will discuss several gauges.
1: Pressure gauge: This says "You got Propane/You are empty" there is very very very little warning.
2: Magnetic or adheasive stick on LCD color strips, these actually work fairly well, using the gas stove boil a pan of water and dump it over the gauge, you should see a clear color line, that's your propane level.
3: Built in float gauge.. THIS has to be built into the tank, Of course if it is not done properly it does not work properly.
4: Scale. Yup, a scale you weigh the tank.
There is a company that makes a simple spring scale that mounts under the tank.