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Due to variables like lighting and the material with which the mirror is manufactured, it can be really difficult to tell whether you are truly touching your reflection or not. You might think you're touching a first surface mirror when you're actually not.

Also, it's possible for a two-way mirror to be a second surface mirror. If other aspects of the situation, like the mirror's setting and lighting, have indicated that what you're seeing is a two-way, don't let the fingernail test be the deciding factor.

While it is not completely accurate, you can use your fingernail to determine if the mirror is a first or a second surface mirror. Simply place your fingernail on the surface of the mirror. When you touch your fingernail to a second surface mirror, you can't touch your own reflection; instead, you will see a gap caused by a second layer of glass over the mirrored surface. When you touch your finger to a first surface mirror, you can touch your own reflection, since there's no additional layer of glass in between. First surface mirrors are very rare, so if you find one there's likely to be a very specific reason and it's very possible that it's a two-way mirror. Second surface mirrors are your ubiquitous everyday mirrors.