The first method is to test the thickness of the mirror with your fingers. I will aim my fingernails at the mirror to see if there is any distance between them. If you see some distance between the two nails, it means that this mirror is a one-sided mirror and an ordinary normal mirror. However, if there is almost no gap between the two nails and they are closely attached together, it means that they are double mirrors. Always use your nails, not your fingertips.
The second method is to tap the mirror to listen to the sound. If you are peeking at a double mirror, your voice is full; The sound of ordinary mirrors is relatively thin. If the sound is hollow and reverberating, it is probably a peeping mirror.
Finally, turn off all the lights in the room, aim the flash of the mobile phone at the mirror, look at the mirror with your eyes and see if there is anything in the mirror carefully. No, it's single-sided. If it is, it may be a double mirror. This method must be accompanied. If there is something in the opposite mirror, it is really terrible to look at it alone. I am too timid to try it alone.
If it is judged to be a double mirror, the opposite person can see us, but we can't see inside. If the hotel room is equipped with double mirrors, be sure to check out in time.