How can you tell a two way mirror?

On a two way mirror, if you put your fingernail up against the glass and the reflection of your nail touches your nail then it may be a 'two way mirror.' On a regular mirror, the silver that reflects is painted to the back of the sheet of glass. Do the fingernail test at home on a regular mirror. Because the silver is on the back of the glass, there should be a gap between your nail and the reflection the thickness of the glass, maybe a 1/4 inch. A two way will not.