There are two main frequency IR emitters are used: 850nm and 950nm. however for detector, the detectable range covers both frequencies.
As we can't see IR, one way to determine whether the IR LED is working is to digitalize the IR light, which means to look through a camera, you will be able to see blue light which you can't see without the camera.
Principle of IR detection:
IR could be detected when a object is close to the detector, as the emitted IR are reflected back.
But it depends strongly on the angle of the object to the detector. Also the strength of the light decays exponentially with distance, which imply IR detection could only be used in short range detection.
Another thing we need to take into account would be the ambient IR. in case of IR detection, we can consider it as noise, and we need to subtract it from our IR detection value to get the actual reflected IR value. (because the ambient IR varies as the robot moves)