If a mirror's surface transparent, translucent, or opaque? How do you know?

If it's a one sided mirror it would be Opaque. Opaque means that zero shreds of light pass through it, so if you hold up a regular mirror because the light shown on it only bounces back instead of going directly through making a hand held, or kitchen, common mirror in general opaque.

Opaque and translucent have the ability to make a shadow transparent will not