Like when trying to choose a ripe melon at the supermarket, there are countless questionable ways to determine how much propane is left in your barbecue's tank. But if you don't want to drag a bathroom scale out to your deck, try Truma's LevelChecker which uses an ultrasonic signal to determine how full or empty your tank is.
You just hold the LevelChecker's tip to the side of your tank, press a button, and it will emit and monitor an ultrasonic blast that tells it if the tank is full or empty in the current location. After a couple of more tests, it's then easy to extrapolate how much propane is left. At just north of $90 it's a little expensive, but far cheaper than the cost of being embarrassed in front of your neighbours when you run out of gas at a BBQ.