Somebody who noticed double-sided mirrors in America has sent this
Beware, especially the girls, of the clever mirrors while in hotel room or using trial rooms, change rooms, bathrooms and toilets anywhere.
Not to scare you, but to make sure that you are aware.
In some hotel rooms, changing rooms, trial rooms, bathrooms and toilets, etc., seemingly a real mirror is hanging on the wall, but actually it is a two-way mirror, that is, some one outside can see you, but you can not see him. It is very difficult to positively identify the surface by just looking at it. So, how do we determine with any amount of certainty what type of mirror we are looking at?
If one places the tip of his / her fingernail against the reflective surface of the mirror and finds a NO GAP between the fingernail and the image of the nail, then it is a GENUINE mirror.
However, if the fingernail DIRECTLY TOUCHES the image of the nail, then it is a TWO-WAY MIRROR. In this case there may be someone seeing you from the other side of the wall or mirror while you can not see him.
The gap is there because silver coating, in case of a genuine mirror, is on the back surface of the glass sheet. Whereas, in case of a two-way mirror, there is no gap because silver coating is on the front surface of the glass sheet.