If there is a gap between the two tips of the fingernails (real and in the mirror), it is a real mirror. If the two tips of the fingernails intersect - without a gap - it is a two-way mirror.
However, in today's information technology age, the use of two-sided mirrors is somewhat outdated. Compared to the tiny cameras that can be hidden anywhere, two-sided mirrors are much more expensive and easier to detect.
Therefore, you should not be too suspicious of every mirror you use in public because it is likely just a normal mirror!