Actually, from the outside, a philosophical zombie would be indistinguishable from a non-zombie. Zombie Chalmers would write books, papers, and opine on the nature of consciousness and his subjective experience exactly as the real Chalmers would.

For a most amusing take on the p-zombie idea, see:

http://lesswrong.com/lw/pn/zombies_the_movie/

(Though humorous, it's the capstone to a series of articles on p-zombies: http://wiki.lesswrong.com/wiki/Zombies_(sequence) )