To check to see if a mirror is an ordanary mirror or a two way mirror place the point of an ordinary pencil to the surface of the mirror and observe the reflection. If the reflection of the pencil touches the pencil, it is a very good bet that it is a two-way mirror through which you can be observed from the other side.

If there is a distance of about 1/8 to 1/4 of an inch between the actual point and the reflected image you can be 99% sure that it is a one-way mirror.

In the cases where you don't have a pencil, a finger can work, it is just harder to determine if it is a thin one-way mirror or a two-way mirror.