Examine the nozzle of the tank carefully. If it has any chips or cracks, discard it.
Look especially carefully at the threads to confirm they are straight and free of chips or pits.
Check the body of the tank. Small rust spots are unavoidable, but large patches of rust could indicate structural damage. Also avoid any tanks with cracks or pits in the surface.
Tilt the tank at 45 degrees. Pour a tablespoon or so of water near the top of the side and let it flow downward. The colder temperature of the propane will form a condensation line. While tilting the tank, if you notice the fill line below the curve at the top of the tank's side, the tank has not been adequately filled. Discard in favor of a tank with an appropriate amount of propane.