1. Observe how the mirror is installed. A normal mirror is hung against the wall, but an observation mirror is set into the wall. If the wall is behind the mirror, it’s probably a normal mirror.
2. Turn off the lights and hold a flashlight or even a small personal flashlight to the mirror. If it is a two-way mirror, the room on the other side will be illuminated.
3. Press your eyes against the mirror and cup them with your hands, creating a dark “tunnel” to block out the light. When you do so, the light in the observation room will be brighter than the light on your side of the mirror, and you should be able to see something beyond the glass.
4. Tap on the surface of the mirror with your knuckle. A normal mirror will produce a dull sound because it is placed in front of a wall. A two-way mirror, however, will produce an open, hollow, and reverberating sound, because there is an open space on the other side.

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