Use my ear with the help of vmpk, it's not difficult to determine the tonality of popular songs. Here are results: http://chyen.twbbs.org/yen/pub/major.html

There is a Matlab toolbox developed by University of Jyväskylä named MIRtoolbox, aiming to analyzing music clips with cutting-edge algorithms. It provides a scientific way to determine the tonality. However, I still like the primitive way :)