To simply check the level of your gas tank, slowly pour hot water down one side of the tank. As a line of condensation begins to appear, you will be able to see what level the gas is at. If a line of condensation does not appear, then the tank is likely to be empty. In order to confirm this, run your hand along the tank and see if there's a change in temperature. A change in temperature will indicate where the gas level is, but no change will further prove that it is empty.