I did a search and didn't find this topic (sorry if it has been covered before, trying to be helpful to newer shooters and save you time searching) so, Here's a tip I found on YouTube).
Good to know thank youBecause the mirror is on the back side of the glass a strong light (flashlight pressed against the glass beside the objective lense) will illuminate the mirror and reflect up into the optic and you will be able to do the same trick with your non-illuminated optic.