Per Snopes.com this misinformation email has been floating around the internet since April 1999.
Two way mirrors are installed INTO the wall, like windows. Ordinary mirrors are hung on a wall. Press your eyes against the glass and cup your hands around them and you'll see into the next room.
Whenever I get an email like this, I Google it, to check for accuracy. Don't take everything at face value.
Posted By: apropo
When we visit toilets, 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 looking at it.
TWO WAY GLASS IMAGE MIRROR IMAGE
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 GENUINE mirror. However, if your fingernail DIRECTLY TOUCHES the
image of your nail, then BEWARE! IT IS A 2-WAY MIRROR!
"No Space, Leave the Place" So remember, every time you see a mirror, do the
"fingernail test." It doesn't cost you anything.
REMEMBER. No Space, Leave the Place:
Ladies: Share this with your girlfriends, sisters, daughters, etc.
Men: Share this with your wives, daughters, daughters-in-law, mothers,
girlfriends and/or friends.