I’ve been training my ears using the Golden Ears program (or more recently, with my personal patch, which you can create on Ableton), for a few months now.
The program starts by boosting or cutting certain frequency ranges in various musical genres, which you try to guess at.
In the process, you build familiarity with the qualities that each frequency range tends to bring out. You also get a sense of how individual instruments sound when boosted at these ranges.
Now, I’m no expert at this. But I’ve noticed certain common patterns in how I perceive instruments, which might simplify the process of classifying what we hear.