Unscrew any hoses or connections to the tank. Lift the tank by the handle and shake. Listen for any liquid stirring in the tank. Any sloshing noise indicates there is propane in the tank.
Read the tank information near the valve and nozzle. LP tanks have weight ratings for full tanks and empty. Get these figures and place the tank on a scale. If the scale shows the tank's empty weight rating, you know the LP tank has no fuel left.
Go to the RV's propane tank gauges. Many modern RVs have monitors on the LP tanks to indicate fuel level for the tanks. Read these gauges much the same as reading a car's fuel levels.
