TexasCHLforum.com

On a related issue you can also see whether you have a loaded magazine on a Glock (at least my Glock 26) by looking above the slide catch on the left-hand side of the gun. Brass will be visible to the eye if you have rounds in the magazine as you look sideways into the gun.

dlh

I've found the loaded chamber indicator is pretty reliable on mine.

I guess one has to be willing to destroy the mirror and what may be behind it in order to use this method.