Quickest way is to find the Radius of Curvature. This is only accurate (in the way I am going to describe) of 1-2" or so depending upon your accuracy. The process is this - Get a small flashlight (pointed at the mirror) and hold it up next to your eye. Find the reflection of the light in your mirror. move around until the light blinks on and off and that is the center of curvature. divide that by 2 and that is the Focal length of the mirror. You will notice that the light moves one way inside of focus and the other way when outside of focus. This is how you can tell if you have gone too far one way or the other. Another way is to set up the focualt test or a Ronchi Grating test. Both of those tests are described elsewhere on the net. Good luck.