Check the tank gauge, if it has one; near 0% usually means empty or nearly empty
Weigh the tank and compare it to the tare weight stamped on the collar
Pour warm water down the side of the tank and feel for a cold line where propane level ends
Use an ultrasonic or magnetic propane level indicator
If appliances stop working or flame gets weak, the tank may be empty
If you hear only a brief hiss when opening the valve, the tank may be empty
If the regulator freezes or the tank feels unusually light, the tank may be empty