I Think a good way of training your ear is by playing and listening to lots of music. As many people have already said, it comes with time, i think you'll find that one day you'll hear a chord and think, 'oh, thats gotta be minor/major'. I found that after time you'll get progressively better at it. maybe you'll start off being able to distinguish chord types, then eventually notes and intervals. It's also good to be able to hear the difference between a scale and an arpeggio.
i wish there was a really fast-track way to doing it, but i dont think there really is, people learn at different paces in different ways. one thing is certain though; you keep practising, and you
Will learn to do it.
best of luck.
