If I connect a headphone to my radio headphone's jack, the radio detects that a headphone is connected and mute the built-in speaker. Similarly if I connect a headphone to my laptop, Windows reports the same thing.
Just curious how the detection is being done? I am pretty sure it does not detect the resistance of the headphone alone because once when I am trying to connect a jack alone with open wires (no actual speaker), it is still detected. I open my radio, observe the headphone jack and do not notice any obvious mechanical switch that gets activated when something is connected to the jack.