I am in the HVAC business and have to get a general idea of tank (refrigerant) levels pretty often. You should be able to tell by feel after the tractor has run for awhile (and still running)as there will be a difference in temperature between the liquid and gas state of the propane. If you are unable to feel the difference,you can pick up an infrared/laser temperature gun pretty cheap nowadays.Just scan the tank from top to bottom unit the temp reading changes and note that line/level.