Read the tank gauge, if your propane tank has one
Use warm water on the outside of the tank and feel for the fill line
Weigh the tank and compare it to the tare weight stamped on the tank
Check the propane supplier app or monitor, if your system has one
Look for a remote tank level gauge on the tank or regulator
Contact your propane supplier for a level check
Use an ultrasonic propane tank gauge, if available