Move up to the mirror and place your fingertip or nail against
it. If the reflection of your finger directly touches your
finger, it is a two-way mirror. If there is about 1 centimeter
of distance between your finger and the reflection, it is not.
Another way is turn the
lights off in the room, then place a bright flashlight against
the mirror surface. If there is a hidden chamber behind the
mirror, the flashlight will illuminate it, and since you are in
a darkened room, you will see the hidden room. Of course, any
mirror hung in front of a wall will be a mirror and pane of
glass installed into the wall, like any other window, will have
window framing.
The recommended
lighting ratio for surveillance purposes is 10:1, with the
subject side being ten times brighter than the observer side.