Hidden Insights on Instagram: "The fingernail test is a common method used to determine if a mirror is a two-way glass by checking the distance between the finger and its reflection. To perform the test, you place your fingertip against the mirror surface and observe the point of contact. If there is a visible gap between your nail and the reflected image, it is typically a standard second-surface mirror where the reflective coating is behind a layer of glass. If there is no gap and your fingern