A method of making an easy open end for a container includes steps of providing a can end blank having an end panel and forming the can end blank in a series of forming operations that are performed with specialized and unique tooling. The can end is preferably shaped during the forming operations so that a major portion of the end panel is curved. Specifically, the curvature is preferably such that a top surface of the major portion is generally concave and a bottom surface of the major portion is generally convex. The end panel is also preferably shaped and constructed so that the curved major portion will invert in shape upon the application of a sufficient predetermined pressure differential between the top and bottom surfaces. A method of testing a fill level of a sealed container is performed in reliance on this “cricketing” or “oil canning” effect.