Check the tank gauge, if it has one
Weigh the tank and compare it to the tare weight stamped on the collar
Use warm water on the side of the tank and feel for the cold propane level line
Look for weak or no flame in appliances
Listen for hissing or gas flow stopping unexpectedly
Watch for appliances shutting off or failing to light
Check for frost or condensation near the bottom of the tank
Confirm with your propane supplier or refill station