A two-way mirror is glass that functions as a mirror on one side and a window on the other. They are also known as transparent, observation and one-way mirrors. All of these names refer to a product called Mirropane, which uses a chemical covering on the first surface of the glass so that one side is transparent. Two-way mirrors are commonly found in police stations, prisons and psychiatric facilities, and less frequently in school classrooms (such as in special education programs). These locations often have an observation room on the other side of the glass. However, two-way mirrors are sometimes found in the dressing rooms of clothing stores, public restrooms, and other inappropriate places where the persons being observed are unaware of the mirror's use. These mirrors usually have a hidden camera behind them, which is much more difficult to detect than a two-way mirror with an observation room. There is no foolproof method that will determine with certainty whether a mirror is a two-way, but the best way to tell is by carefully examine the glass using the steps below.
Pay close attention to how the mirror is installed. Normal mirrors are hung in front of the wall, but a two-way mirror with an observation room must be set inside the wall. If the wall is clearly behind the mirror, it is probably not a two-way mirror. The exception is when there is a two-way mirror with a camera behind it.
Cup your hands against the mirror and peer through them. This will block out the light in your own room and make the light more obvious in the observation room behind the mirror (if there is one).
Turn off the lights in the room you're in and shine a flashlight on the mirror. If there is an observation room behind the mirror, it will be illuminated. If you do not see an observation room, you may want to shine the flashlight in various places on the mirror to look for any small hidden cameras.
If you do not have a flashlight and are finding it difficult to see into the mirror, you can try rapping your knuckles against the glass and observing the sound you hear. A normal mirror will make a dull sound because it is hung up against the wall, but a two-way mirror will produce a hollow sound due to the opening on the other side. Keep in mind that hidden cameras only need a small amount of space cut away from the wall, so a two-way mirror with a camera and not an observation room behind it may not make a reverberating sound.
A less reliable way to spot a two-way mirror is with the "fingernail test." Place your index finger against the mirror so that your nail touches the glass. If it is a normal mirror, you should see a gap or space between your fingernail and its reflection. A two-way mirror would not leave this space.