Headphone Wave Sensor

I wonder what people wearing headphones are listening to.  It would be fun to try to make a prediction and check to see if I was right.  I feel like half of the time I would be able to guess correctly and the other half I would be way off.

I need a device that can pick up electromagnetic something-or-another from someone's headphones and stream their music to headphones of my own.  Or, another option would be just to gather the name of the song, then automatically look up the song and play it from my phone.  That way I could just walk somewhere and have a fresh new playlist of music chosen for me based on the preferences of random strangers.