i took this from one of "sluggo's" links:

Sound and light act a lot alike above 400Hz, so you can use a mirror and simple geometry to find these unwanted reflections. Invest about $30 in a 2’x2’ plastic mirror (no frame). You can get one at a plastics store. Have someone sit in the listening position while you hold the mirror absolutely flat against the sidewalls and ceiling. Slide the mirror all around to see if the listener can see the speaker components (not the side or top of the speakers) in the mirror. Every time the speaker is seen in the mirror, that is a first order reflection point. You will want to put a treatment in that place. Remember that you must keep the mirror perfectly against the boundary surface while doing this test. It’s just like billiards; the geometry needs to be exact. You will probably be able to outline one large area on the sidewalls that shows the reflection of all three front speakers.

is it safe to assume,that if,you can't see the drivers,in the mirror,you have good sound?
no i wasn't referring to my rear view mirror!
i'm kinda curious.