How many times have you walked in to a trial room nonchalantly, totally unaware that a lecherous idiot is watching you from the other side, courtesy a "two-way" mirror installed? Follow these simple tests to tell whether the mirror in front of you is a normal one or a "peek-a-boo" session:

Peer through the glass

Try blocking as much light as possible as you peer through the mirror. If you see it leading to another room, then you're in for trouble!

Source: The Macular Degeneration Guide

Shine a light

Try turning off all the lights in the room and use your phone to flash a light onto the mirror. If it's a two-way mirror, the other side would be illuminated. 

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Tap the mirror

Tap on the surface of the mirror with your knuckle. It should produce a flat sound (because it's placed on a wall). A two-way mirror, on the other hand, would produce a hollow sound because of the open space on the other side. 

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The fingernail test

Though not always accurate, you could perform a fingernail test to ascertain the presence/absence of a two-way mirror. Place the tip of your fingernail against the mirror's reflective surface. If there's no gap between your finger and the reflection; chances are, someone's playing "peek-a-boo" with you. 

Source: Mao Mao Pet

Beware.