I can tune a guitar quickly by ear too. I learned that before we had electronic tuners (as most of us did). And I can tell where the chord changes come in on-the-fly, and what chord to go to. And when someone is out of tune and by how much and usually which string. And I can sing a harmony part to your melody by ear right away. And I don't have perfect pitch. If I have anything, it's relative pitch.

If you've got perfect pitch, I can play a note, any note, without you seeing it and you can tell me with no hestitation which note I am playing. Nice parlour trick, but not much practical application I've seen.

I know when my strings are worn out when the overtone harmonics don't match up. The strings are in tune (according to the electronic tuner) but they still don't sound right.