Different magazines and the Internet have descriptions of probes for checking the operation of remote controls on infrared rays. Usually, this is a simple circuit from a photodetector or phototransistor and LED, plus a power source. If you direct the serviceable remote control to this probe and press the button, the LED flashes.But, in fact, no probes are needed - the glow of the IR LED can be seen with your own eyes. True, this will require any mobile phone with a camera. We turn on the camera on the phone, point to the remote control, we see the remote control on the telephone display. Push the button ... And, lo and behold! The display shows the glow of the IR LED in gray-blue.
NOTE: For a proper diagnosing a oscilloscope and specializaed tools are needed..................