Distinction

• Also known as one-way, transparent or observation mirrors, two-way mirrors are deliberately designed to function as both a regular mirror and a window.
  
  

Telltale Installation

• A two-way mirror is inset rather than hung upon a wall to allow see-through visibility. Without a solid wall behind it, the two-way mirror will produce a hollow sound rather than a thud when you knock on it.
  
  

Hard Evidence

• Turn off the lights and hold a flashlight or laser pointer against a two-way mirror, you will see its beam pierce through to the observation room. Press your face to the mirror, cupping your hands over your eyes to block out light, you will discern something beyond the glass.
  
  

Misleading Test

• The widely propagated but unfeasible "fingernail test" would get inaccurate results. The two-way mirror's side that would reveal a gap between your fingernail and its reflection faces the observation room, not you.