Turn off the light
Take a video of your room with a flashlight on your phone. At first, do not look at the phone screen but around instead. In the light of the flash, you can notice glare from the lenses of a hidden camera. If you didn’t notice anything, turn your eyes back to the smartphone screen since many cameras illuminate the room in the infrared range. The eye does not see this illumination, but the matrix of the smartphone can handle this. Bright white dots are these sources of light.