The susceptibility of austenitic stainless steel is tested by inputting a highly magnified image of the polished and etched steel surface through a microscope, a CCD camera and an input device into an image processor. The image processor is set up to identify grain boundary locations in the image, take width measurements across the grain boundaries, based on luminance distribution, and calculate average and maximum values for the measured widths. These calculated measured values are compared with experimentally-determined reference values -1 to 1.5 .mu.m for mean width and 2 to 3 .mu.m for maximum width - and a susceptibility status determined accordingly. The status can then be output using a display e.g. screen or printer.