Interesting: How to detect a two-way mirror

http://urbanlegends.about.com/library/blmirror.htm

1) Pressing your finger against the mirror would look different compared to the two-way mirror.

2) The molding of the mirror, is that like a window.

3) Hitting and making a sound, where a two-way mirror would sound like hitting a window.

4) Turning off the lights in the room, or cupping your hands while looking through them, to cut out the light from behind you, would reveal the other room.

Hmmm, good to know.