I know this is late, but I figured I'd make a comment...
The "fingernail test" does
not actually tell you if the mirror is a two-way or not, it just tells you which side the mirror is coated on. The coating itself is what decides whether it's a two way mirror. The vast majority of plate glass mirrors (like the ones in your vanity at home or in the public restroom at the rest stop) are coated on the
back side, simply to protect the coating; front-side mirrors are typically only used in telescopes or some types of laser systems (like those nifty barcode scanners at the grocery store). Holding your fingernail to the surface of a back side mirror will give you a gap between the reflection and your fingernail (when viewed from the side) because of the thickness of the glass; the coating on the back of the mirror could still be a two-way coating. In fact, most two-way mirrors have the coating sandwiched between two plates of glass, again to protect the coating; regardless of which side you were on, you'd still see a gap between your fingernail and its reflection. The only way to tell for sure is to go behind the mirror and look back through it, or wait until someone turns on a light in the room behind the mirror. What's that? Can't get behind the mirror? What are you worried about, then?
This whole two-way mirror thing smacks of urban myth-hood; except for a few types of motels, there simply isn't enough room to get behind mirrors in most public places like restrooms. That's why plumbers make so much money: there's just no room to work back there. Besides, with modern mirror coatings
any mirror will become a "two-way" mirror if a bright enough light is placed on the other side.
Cheers!
Jim
Courage is not the absence of fear. Courage is what you do in spite of your fear.
Every man has fear. Any man who has no fear belongs in an institution. Or in Special Forces.