Any good machine shop has an ultrasonic thickness checker. It is used in other industries for non-destructive thickness checking of metals, such as the thickness of a ship's hull. It isn't nearly as "black magic" as it might sound. It's just a small hand-held tester with a hand-held probe that you apply a little grease to in order to make a good connection, hold it up against the metal to be tested and it sends ultra-sonic signals through the metal that bounce back when they hit the other side of the metal and result in a thickness readout on the window of the unit. The machinist would check the thickness in several different locations in each cylinder.