Magnetic particle testing is a widely established non destructive method for the detection of surface breaking flaws as well as sub-surface flaws with depths of 2-3 mm.  The method is accomplished by inducing a magnetic field in ferromagnetic materials.  The surface is coated with a white contrast paint and over this paint iron particles suspended in liquid are applied.  Surface and near surface flaws produce magnetic poles and iron particles are attracted and concentrated, producing a visible indication of the defects on the surface of the material.  The length of an indication can be accurately determined, but the depth cannot be determined with this method.