One-way mirrors are synonymous with detective scenes. A criminal sits in a bright, white room with a big mirror on one of the walls, and on the other side cops sit in darkness to observe. That's all well and good, but there are plenty of places where a one-way mirror would creep us out. How can you tell the difference?
How one-way mirrors work
One-way mirrors are basically windows. You know how when you look out of a window in your house you can sometimes see your own reflection overlaid with what you see outside? When it's dark out and you have the lights on, the effect is further exaggerated; you can see more of your reflection and less (or any) of what is outside. If you were to go outside, though, you could easily see inside the house because the lights are on.
One-way mirrors operate on this basic principle. Bringing it back to the police station scenario, the setup is designed to make it really bright "inside the house" - where the criminal is - and really dark "outside the house" - where the cops are sitting. The criminal will only be able to see his own reflection and nothing coming from the other side. From the cops' perspectives, the room they sit in is so dark that they can easily see into the lit room just as if they were looking into a house at night from the outside.
A normal mirror
Normal mirrors use a reflecting surface to bounce incoming light back and do not let any light pass through; this reflecting surface sits on a level below the top layer. Simple enough.
Telling the difference
So let's say you are in a dressing room and paranoia sets in that you are being watched from behind the mirror! Simply place the tip of your finger against the mirror and look at it from the side. With a true mirror, you will see a gap between your finger tip and its reflection. This is because of the gap between the top layer and reflecting layer. However, if you can see the tip of your finger touching its reflection then you should probably pick a different dressing room (and alert the authorities).