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[Skvez]

A two way mirror works by being partially silvered rather than totally silvered so the majority of the light is reflected but a little leaks through to the far side providing a much dimmed view.
Turning a window into a one-way-mirror can be done by sticking a very fine film on the surface and this would be revealed by the "nail vs reflection" test but there are other one way mirrors that it wouldn’t detect.

Most one way mirrors are detectable by the fact that the reflection is slightly but noticeably dimmer than it should be (as some of the light has to pass through). You'd need a very dark room on the far side to have a useable one way mirror that reflected enough light to not be noticeably dimmer and still be able to see the greatly dimmed image from the 'dark' side.

When I'm in a changing room, if I suspect the mirror of being one-way I hang my coat over the mirror while I'm trying on the clothes rather than on the hook on the door. If this is impossible (such as in cases where the mirror is flush with the wall) then I face the door not the mirror while changing. This is not natural as we tend to face away from the door when getting changed.