You want there to appear a "space" between your fingertip and the mirror, if touching it directly. If there's no space, then say cheese.

This is ancient advice for two-way mirrors, IIRC nowadays even legit mirrors can reflect directly from the front instead of the back. In this age of spy cameras this is mostly irrelevant.

Another tip was shining a bright light to illuminate the supposedly dark room on the other side, which again, would be way more expensive than a smoke detector spy camera.

It would be the other way around, if at all.

"First-surface" mirrors where the reflective layer is on the front of the glass are quite fragile, so wouldn't typically used for residential applications (you'd remove the reflective coating by cleaning it).

A regular mirror has the reflective surface on the back of the glass (which is then is further coated with a protective paint), leading to the effect you describe.

I don't however know enough to say one way or the other whether a surveillance mirror would becessarily be a first-surface mirror.

think if the mirror is legit you’ll see the reflection directly on the surface, if it’s two way the reflection will look like it’s inside the glass.

scientific use mirrors are front mirrors, the metal is deposited on the outer surface. They are delicate and difficult to clean.

Every household mirror is rear coated, and then painted to protect the metal from oxidation.