For critical erbium-doped fiber amplifier (EDFA) design, e.g., gain tilt optimization in WDM booster amplifiers, knowledge of the gain distribution within the active fiber can present a valuable information. Among the different techniques to evaluate the distributed gain in active fibers, the technique of optical frequency domain reflectometry seems most promising as it is a non-destructive measurement method well matched to the task due to its dynamic range, resolution, and range. Moreover, background light from ASE or residual pump light is strongly rejected due to the coherent detection scheme employed. Using different erbium-doped fibers with strongly varying doping levels and confinements, we demonstrate the excellent accuracy and reproducibility of the technique.