Get a gauge.
There are all sorts of devices available. There are inexpensive paste on strips that will display the temperature change at the the level of your liquid propane. There are expensive hand held ultrasonic tools that can detect where the liquid or gas is inside the tank. And in the mid-range, you will find mechanical or electronic tank scales. These may be built-in or separate from the grill and can be calibrated to tell you how much propane you have left with an analog or digital display.