here's what's needed and it's not easy. You have to distinguish between the "voicings" of the chords. These are the pitches that are in the chords. You have to begin to recognize the pitches of the tones in the chord. For example, a major chord is built on the first, third and fifth note of a scale. Major chord=1,3,5. Now, you have to play a major chord and see how these pitches are distributed then you have to identify the 1's,3's and 5's by sound.

Eventually you can learn to hear the various chords by their "voicings" or tone-pitches.

If you are a studious musician, you can learn to identify any chord in this way. Sometimes it takes years but I have taught people to do this to some degree in weeks.

Once you recognize the "voicings" of the chords, then you slap a capo on, the "voicings"
remain the same although the key has changed. When I teach this, I have an excercise
which I call "singing chords". I have the student identify the pitches of the chords by numbers. Then it becomes more available to translate the pitches into actual musical notes. Unfortunately, you gotta' be there.

Next, you have to identify the position of the chords in a progression. That is done numerically using roman numerals. Eventually, you can abstract a chord progression this way from any particular key and apply it to any key.

The combination of identifying the "spelling" of the chord with the "position" of the chord in a progression enables you to use the capo and select keys efficiently.

Frank Hamilton