I recently tried out an app called Perfect Ear. You can test your recognition of intervals either from hearing audio or reading dots. It also has exercises for scales and chords.
The experience reinforced what George has said in previous threads about reading, hearing, memory, and physical control of the instrument being distinct processes. I play three instruments and have learned some tunes by ear, some from sheet music, and some from a combination of reading lead sheets and listening to recordings. For me, touch, hearing, and visual cues are complementary parts of learning music, but its interesting to isolate those elements to see what needs the most work.
If you are puzzling over muscle memory, playing by ear, or sight reading here are some other experiments to try:
-When you look at a familiar piece of sheet music, can you hear the tune in your head? Can you visualize the sequence of key presses on your accordion? If you press the keys without moving the bellows, is it harder to play than when you have audio feedback? What if you block your minds ear by saying the rhythm with a monotone rat-tat-tat as you press the keys?
-Can you sing the whole piece? Can you do it without sheet music or charts?
-Can you take a piece of music that you learned on another instrument and play it on accordion?
-Can you transpose a simple, familiar melody to a more difficult key?
-Can you read something in an unfamiliar clef? I tried playing along with my sons beginner viola material. I cant recognize the notes on Alto Clef, but I could sometimes identify the intervals well enough to follow along.
-When you hear a three-chord folk song, can you recognize I, IV, and V chords well enough to follow along using Stradella Bass? Does it matter what key the song is in?