How can I test if my cookware has a flat bottom?

Turn the pan upside down on the countertop and place a ruler flat against the bottom of the pan—the bottom of the pan and the straight edge of the ruler should be flush against each other. Another test is to place 1 inch of water in the pan, bring it to a boil, and observe the location of bubbles; good flat cookware will have an even distribution of bubbles over the entire bottom surface area. (Page 7)