You can estimate how much gas you have left in your propane tank by heating up some water to a boil (you can do this in the microwave or inside). Make sure you have enough hot water to gently heat the SIDE of the propane tank in question. Tilting the tank on a bit of an angle, pour the hot water up and down the entire side of the tank from bottom to top.
The level of liquid propane inside can be determined by feeling the new temperature of the tank with your hand.
Where your hot water has succeeded in heating the tank is where it’s empty. Where the tank is noticeably cooler is where you have some remaining liquid propane.
If there is no noticeable difference you’re probably all out.
Be careful! Do at your own risk!