Turn off the lights in the room you’re in and shine your phone’s flashlight on the mirror. In a two-way mirror you will likely be able to peer through to the other side. If there is an observation room behind the mirror, it will be illuminated. If you do not see an observation room, you may want to shine the flashlight in various places on the mirror to look for any small hidden cameras.
3. TAP THE SURFACE
You can also tap on the surface of the mirror with your bare knuckle and listen to the sound it produces. A normal mirror will usually produce a dull sound because it is placed in front of the wall. However though, a two way mirror will make a more hollow and open sound that reverberates when tapped against and that is due to there being an open space on the other side of it.
4. LOOK AT IT
Cup your hands around your eyes and peer very closely into the mirror. Often you will be able to get a glimpse of the other side if it is a two-way.
So remember, every time you enter in changing rooms, do these simple mirror tests to check for two-way mirrors.