Detect a two way mirror. How?

Here something cool that I found on net. There are many places you vist, where you are being watched. Many places like hotels and restaurants do spy on private activities of people.
So how do you detect that you are being spied or not. The most common trick to spy on someone is to place a 2-way mirror in the room. A 2-way mirror looks like a simple genuine mirror, but its a plain glass from other side.
Heres a trick to identify a 2-way mirror. 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, IT IS A 2-WAY MIRROR! (There is someone seeing you from the other side). So remember, every time you see a mirror, do the "fingernail test."

-Bugs!