Infrared LEDs -- light emitting diodes -- are used in many remote-control systems, such as television remotes and garage door openers. Infrared has a longer wavelength than visible light, so you cannot see it with the naked eye. This makes it harder to diagnose problems with infrared LEDs, as you cannot see whether the LED is lit or not. The solution is to use a digital video camera or a video-enabled cell phone that can "view" the infrared light and display it on the viewfinder as a purple glow.
Put fresh batteries in the device being tested. This rules out flat batteries and enables you to see if the infrared LED is faulty or not.
Turn on the digital video camera. If using a cell phone, switch the phone to its digital video mode.
Point the digital video camera or cell phone at the infrared LED. This is normally the area covered with black shiny plastic on a remote control, for example.
Press a button on the remote control. If the infrared LED is working, it will appear on the digital camera's viewfinder as a purple glow.
If the infrared LED is faulty, you can purchase replacements from most electrical stores or online.