I learned by ear when I was 12 years old. After I learned some chords I would play along with some records (Kingston trio, actually) which were loaded with 3 chord songs. I would do this by sliding my capo up the neck to first establish what key a song was in. then I would try to play along.

Then (and this is where the ear comes in) I would LISTEN where the drone pitch changed. The drone pitch is the same as the chord and when you heard the change, this is where the chord changed (usually to a 4th chord or a 5th chord)

I would literally hum the drone pitch with the chord until the drone pitch (and the chord itself) needed to change. It works.

Hum the pitch of the chord. Listen for the pitch to change. The chord changes there.