&gt; For line inputs, as they&#x27;re usually AC-coupled<p>Headphone amplifiers often have DC-coupled 10k potentiometer from input to ground with washer feeding a DC- or AC-coupled gain stage. So much for AC&#x2F;DC detection.<p>I think impedance measurement is used in practice. I can hear a quiet beep when I connect headphones to one of my laptops, for example.