Open your smartphone's Camera app.
Switch to the front-facing camera. If the camera isn't showing your face while you hold the screen side facing you, tap the "Rotate" icon (which usually resembles a circular arrow or two) to flip it.
(You can't perform this process with the rear camera.)
Verify that your smartphone can see infrared light. In order to search for hidden cameras, your smartphone's front camera cannot have an infrared filter. You can determine whether the camera has an IR filter by using a TV remote:
Point a remote control at the camera.
Press any button on the remote control.
Look for a flash on the remote control's front light.
Turn off the lights in the room you want to scan. In order to scan for infrared light, you'll need to have the room as dark as possible.
{ If there are any other lights in the room (e.g., night lights, power strip indicators, etc.), unplug their sources if possible. }
Use your smartphone's camera to look for flashing lights. With your phone's screen facing you, rotate while looking for flashing spots. If you see a flashing area, it's most likely a hidden camera's IR light.