I don't understand why yet. Why is it important to you to know whether someone is listening to music or not? Why do you need to be able to tell at all?
> I sometimes wear headphones without listening to music because I don't want my train of thought to be interrupted.
totally understandable as the "right to be left alone" or privacy is essential but undeniably curtailed when in public.
i hope you can appreciate the collective imperative here and acknowledge that it would be super fucked up if everyone would be wearing headphones in public.