To know that your inspection device/method is giving you reliable results, you can reference the specification sheet that came with the instrument. The unfortunate truth, however, is that this number often has little direct correlation to the results generated during the measurement of your production workpieces. By far the best solution for evaluating a CMM is to use a combination of an International Standard Organization standard (ISO-10360) and a gage repeatability and reproducibility (GR&R) test. The goal of a reduced gage variation is to ensure that you maintain as much of the available manufacturing tolerance as possible. When there is a high GR&R, you effectively reduce your manufacturing tolerance, thus making it more difficult to manufacture your workpieces.