At least some MBP's and iPhones do this too (e.g. mine). In my opinion (though less reliable than the sibling's because I'm an ME not a EE) impedance detection

At least some MBP's and iPhones do this too (e.g. mine). In my opinion (though less reliable than the sibling's because I'm an ME not a EE) impedance detection is probably the easiest way to do this.