When we visit bathrooms, hotel rooms, changing rooms, etc., how many of you know for sure that the seemingly ordinary mirror hanging on the wall is a real mirror, or actually a 2-way mirror (i.e. they can see you, but you can't see them)?
There have been many cases of people installing 2-way mirrors in female changing rooms. It is very difficult to positively identify the surface by just looking at it. It's time to get paranoid. So, how do we determine with any amount of certainty? Just conduct this simple test:
Place the tip of your fingernail against the reflective surface and if there is a GAP between your fingernail and the image of the nail, then it is a GENUINE mirror. However, if your fingernail DIRECTLY TOUCHES the image of your nail, then BEWARE, for it is a 2-way mirror!
So if not at home and changing before a mirror, do the "fingernail test". It doesn't cost you anything. It is simple to do, and it might save you from getting "visually raped"!
source post and more… [comments by Peter Kohler]