There are two ways to test if a mirror is see-through:

Try to peer through the glass. Press your face up to the mirror and cup your hands around it, creating a dark tunnel to block out as much light as possible. When you do this, if the light in the observation room is at all brighter than the light on your side of the mirror, you should be able to see something beyond the glass.

Conduct a simple “fingernail test.” Place your fingernail against the reflective surface in question. If there’s a gap between your nail and the mirror, it’s most likely a genuine mirror.