Many applications of optical fiber, such as specialized fiber Bragg gratings (FBGs), require high uniformity of a fiber's refractive index (RI) along its length. We show here that the mode effective index of most fibers is not constant, even on a short length scale. To help improve fiber manufacturing and selection, we demonstrate a technique for characterizing a meter length single-mode optical fiber's effective RI over a centimeter scale with a precision of 3×10-6 RI units (RIUs) and an absolute accuracy of 2×10-4 RIU. By writing several weak probe FBGs as frequency references and then measuring the frequency deviation of these probe FBGs along the length of the fiber with an optical frequency domain reflectometer, the RI distribution of the effective mode index may be found. We validate our measurements on reference and fibers under test with theoretical simulations.