The easiest way I know is to take the mirror out of its cell, and using a distant light source, located along the central axis of the mirror, form the sharpest image of the light source that you can on a screen located just off- axis of the mirror. This distance is the radius of curvature of the mirror, which is exactly twice the focal length. When grinding a mirror, this method gives you a pleasant check on your progress in reducing the focal length from 'infinity' to the desired 'f/' number.