Returning from my first shopping trip without my DH, I said to him, "Do I look pale or sick or something?" He said, "No, why?" I said, "Because three people asked me if I was alright today." He cracked up.
I always answer now, "I"m fine, thanks. How are you?" Most seem to like the fact that I am inquiring about them, and answer me. Only one ever seemed a bit surprised that I gave them a full answer.
In the US, I always answered the "How ya doing?" with the same, "Fine, thanks. How are you?"