I'm not sure if this is the case for all phones, but I think the front-facing camera on some phones can show infrared light. If you're concerned about hidden cameras in a room, you could try waiting until it's dark and then looking at the room through the front camera viewfinder. Any cameras—at least those with night vision—will show up as lights. (My understanding is that the rear camera won't show IR light; that can degrade photo quality, and since most people use the rear camera for photography, that camera has an IR filter.)
To test whether your phone camera can see IR, go into a dark room and point a standard TV remote at the camera while pushing the buttons. If you see a light (might be dim), your camera can see infrared.