While this isn't exactly playing by ear, it is a good cheat that gets you going on new songs. Observe another playing the same instrument as you and copy the chords that s/he plays. Or observe another good musician on another instrument, learn to recognise the chords that s/he plays and finger that chord on your instrument. This worked well for me when I was struggling along on a banjo and could watch the guitarists, and play the same chords they were using on my banjo.

Dave