text: There really isn't such a thing as a one-way mirror.  What is referred to as a one-way mirror is simply an arrangement involving a semi-silvered mirror and two rooms, one kept dark, and one kept very bright.  The mirror is set up to reflect most light, but also let a small amount of light pass through to the other side.\n\nIf you're in the brightly-lit room, and you look at the mirror, you see the reflections of the bright objects in the room.  The things in the dimly-lit room don't shine brightly enough to be noticed against the background of the bright reflections.\n\nIf you're in the dark room, there are no distracting reflections from your side of the room, so you see the small amount of light that passes through the mirror.\n\nIt's like mirrored sunglasses, you can see out, because between the glasses and your eyes things are quite dark, but people on the other side of the glasses just see reflections.  Turn your mirrored sunglasses around, and they still work the same way (though the colour of the reflections might change as seen from the outside if the sunglasses are tinted as well as mirrored).\n\nIf you were to take the "one way mirror" out of its frame and turn it around, it would not reverse the direction of visibility, because whether or not you can see is a function of the lighting conditions, not any built-in direction in the glass.  Conversely, if you were to turn down the lights in the bright room, and turn on a light in the formerly dark room, people would be able to see through the mirror to the side they were supposed not to see.\n\nNow, on to the question.  There have been various suggestions relating to the visible gap between a finger touching the mirror and its reflection, but these all just measure the thickness of the glass.  The mirror part is not the glass, it's the metal coating on the back.  The glass is there to protect the metal, give it shape, and be smooth.  All common household mirrors are built this way, so all of them will appear to be "one way glass" by the test provided.\n\nThe best way to check is to get right up to the mirror, cup your hands around your eyes to block out light, and shine a bright light at a part of the mirror which is outside the piece you've blocked off with your hands.  If it's a one-way mirror, you will be able to see through the glass and will see the spot your light makes on things in the room behind.