Have you ever been in a bathroom, dressing room or another private area with a mirror and had that feeling that someone is watching you?
  1. Observe how the mirror is installed.
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    Notice if the mirror seems to be hanging on the wall or if it's part of the wall itself. If it appears to be hanging, try to look behind it and see a wall. If the mirror seems to be part of the wall itself, there's a good chance it's a two-way mirror, which must be set into the wall rather than hung onto it. That way, people standing on the other side of the wall can observe someone looking in the mirror.
  2. Check out the lighting.
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    Look around and determine whether the lighting seems extraordinarily bright. If so, you might be looking at a two-way mirror. However, if the light in the room is relatively dim, and you can't immediately see through the mirror, it's probably just a standard mirror. For a two-way mirror to be effective, the light on the mirrored side needs to be 10 times brighter than the light on the other side. If the lighting is any dimmer, it's possible to see through the glass to the observation area.
  3. Consider where you are.
    If you're in a public place and in an area you'd expect privacy, such as a restroom, it's unlikely and illegal to have a two-way mirror. On the other hand, two-way mirrors are frequently used by law enforcement. Most states have passed additional legislation preventing the use of two-way mirrors in rest rooms, locker rooms, showers, fitting rooms and hotel rooms. If a location has chosen to use two-way mirrors or surveillance, they are required to post signs that notify you.
  4. Try to peer through the glass.
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    Press your face up to the mirror and cup your hands around it, creating a dark tunnel to block out as much light as possible. When you do this, if the light in the observation room is at all brighter than the light on your side of the mirror, you should be able to see something beyond the glass.
  5. Shine a light on it.
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    If you're still not convinced, turn off the lights, then hold a flashlight to the mirror (it can even be the "flashlight" on your smartphone). If it is a two-way mirror, the room on the other side will be illuminated and you'll be able to see it.
  6. Sound it out.
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    Tap on the surface of the mirror with your knuckle. A normal mirror will produce a dull, flat sound, since it's placed in front of a wall. An observation mirror will produce an open, hollow and reverberating sound because there is an open space on the other side. The sound of tapping a two-way mirror has also been described as bright or sharp as opposed to a thud from an everyday mirror.
  7. Perform the fingernail test. While not completely accurate, you can use your fingernail to determine if the mirror is a first or second surface mirror.
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    Place your fingernail against the surface of the mirror. When you touch your fingernail to a second surface mirror, you can't to touch your own reflection; instead, you will see a gap caused by a second layer of glass over the mirrored surface. To a first surface mirror, you can touch your own reflection, since there's no additional layer of glass in between. Second surface mirrors are your ubiquitous everyday mirrors, but it's possible for a two-way mirror to be a second surface mirror.
  8. Consider the extreme measure of breaking the glass.
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    If it's a regular mirror, it will shatter and you'll see the mirror's backing or a solid wall. If it's a two-way mirror, you'll see the room behind the mirror. You should probably only consider this option if you feel threatened or are in danger. Breaking the glass will cause damage and create a safety hazard.
  9. Warning: No test for a two-way mirror is definitive.
    There only needs to be a very small opening in the wall for a hidden camera with a fisheye lens and there won't be any giveaway illumination on the other side, or any hollow sound or anything to see with your cupped hands. Even if the mirror is a normal one, there are many other places to hide observation devices.