If I have a tune fully in my head, fully hummable, fully whistleable, I can play it straight away on the harmonica. I never have to search the instrument for the right notes. Naturally, it's a bit raw to begin with, not very subtle, but that's what I work on. I can't learn a tune from printed music even though I understand printed music and can follow an orchestral score. I need to hear a tune a good few times and feel motivated, then I've cracked it.