A molding technique is utilized to form the cooling channels in an airfoil for a gas turbine engine. The cooling channels are formed by lost core mold cores, which are leached away after metal has been molded around them. At least one of the mold cores is provided with a cone which will extend into the airfoil. The cone forms an opening in a wall of the airfoil, and the size of this opening is indicative of the thickness of the wall. By comparing the size to expected sizes, a determination can be made of whether the wall is of an acceptable thickness at locations where it is difficult to otherwise measure the wall.