How to Tell if a Mirror Is Two Way or Not ~ Do you know how to determine if a mirror in restrooms, dressing rooms, even motels is 2-way or not? Here's a simple test to determine the type of mirror it is. Place your fingernail against the mirror and if there is a GAP between your fingernail and the image of the nail, then it is a GENUINE mirror. However, if your fingernail DIRECTLY TOUCHES the image of your nail, then BEWARE! IT IS A 2-WAY MIRROR! "No space, leave the place"