Lens detection or signal detection
These find different cameras and it is worth understanding which you need. A lens detector is a ring of red LEDs around a small viewfinder. You hold it up to your eye, look through the viewfinder and sweep the room slowly. A camera lens bounces that red light straight back at you and shows up as a bright point that blinks as you move your head, whether the camera is transmitting, recording to a card or switched off. It is slow, because you are scanning the room by eye, but it finds cameras nothing else will.
A signal detector sweeps for transmissions instead and finds any camera streaming over WiFi or 4G in seconds, much faster, but blind to a camera recording locally. Devices that combine both are the practical choice for most people.
How to sweep a room
Work methodically rather than quickly. Check anything with a view of the bed or bathroom door first: smoke alarms, clocks, chargers, air purifiers, power outlets and ceiling fittings. Turn the lights off for lens detection, reflections are far easier to see in a dark room.
How to check a room yourself, before you buy anything
Most people searching for this want a method, not a product. Start with the lights off and the curtains drawn, then look along the room at eye level and slowly sweep a torch across every surface. A camera lens returns a small, hard, bright point that moves as you move, quite unlike the soft glow of a plastic reflection. Take your time on anything with a hole in it, no matter how small: a lens the width of a pin is normal now.
Then look at the room the way whoever placed it would have. A camera needs a power source, a sight line, and a reason to be there. Work along the mains sockets, the shelf facing the bed, the smoke alarm above the doorway, the clock on the bedside table. Anything pointed at a bed, a desk or a doorway deserves a second look, and anything that has been moved since you arrived deserves a third.
Check the WiFi list on your phone as well. A camera streaming over WiFi has to appear on a network, and an unfamiliar device name in a place you are staying is worth asking about.
Where they actually get put
In rentals and hotel rooms the recurring places are smoke alarms, air vents, mains adaptors and USB chargers left plugged in, digital clocks, TV units, power boards, and decorative items on a shelf at bed height. In changing rooms and bathrooms it is more often a hook, a vent, a shampoo bottle or a hole drilled through a wall panel. In workplaces it is chargers, desk clocks and speakers, because nobody questions those.
What a detector finds, and what it does not
A lens detector works optically. Red LEDs around the eyepiece light the room, you look through the viewfinder, and any lens pointed your way throws the light straight back as a bright red dot. Because it reads the glass rather than a signal, it finds a camera whether it is transmitting, recording to a card, or switched off entirely. Its limit is line of sight: you have to be looking roughly along the axis of the lens for the reflection to come back, which is why a slow sweep from several positions in the room beats one quick pass.
An RF detector works the opposite way. It finds anything transmitting, including cameras hidden where no lens is visible, but it is blind to a device that is recording locally or is powered down. Between the two, the optical pass catches the recorder and the RF pass catches the streamer, which is why most units here do both.
How thorough is thorough
A careful sweep of a hotel room takes ten to fifteen minutes. A house takes an evening. If the concern is serious rather than precautionary, the honest answer is that a physical search of the likely objects, opening them where you can, finds more than any handheld device on its own.
Related
For a tracker hidden on a vehicle, see GPS tracker detectors. For microphones and recorders, see listening device detectors.
Common questions
Will it find a camera recording to a card? Only lens detection will. A signal detector needs the camera to be transmitting.
Does it work through walls? Signal detection can pick up transmissions from an adjoining room. Lens detection needs line of sight.