Hold the objective end close to a mirror under a good light. I used my bathroom mirror. You'll need to tweek the scope/mirror distance and your eyeball/occular lens distance. Tweek distances until you see two crosshair images, one dark and centered and one lighter and not centered. |
The gif image on the right shows how one of my pistol scopes looked through a mirror. The scope was adjusted way high and way to the left. To return the scope to optical zero, I simply adjusted the knobs until the off center crosshair aligned with the center, darker crosshair.
Don't ask me how it works 'cause I don't know. I just know it works and works fast and easy. It works with fixed, variable, and even pistol/LER scopes.