Not advanced science, just a little application made possible by technology. To check a remote control, you need a digital camera, a cellphone with camera will do. Turn the camera on, aim the remote at the lens of the camera, and press the button you want to test. If the button works, you see a flash on the display of the camera.
The infrared light of remotes is invisible for the human eye, but the camera's image sensor picks it up, and shows it as white light on the display.