In The Walking Dead, both the show and the graphic novels, it is established that the walkers determine prey from other walkers by sense of smell.
However, we see some pretty far decayed walkers, some of which are missing large parts of the face (I am pretty certain that I've seen some missing noses). Others seem to be missing most, if not all, of their respiratory system:
(note the exposed rib cage above)
It seems pretty established that walkers only need a head in order to survive. In the graphic novel, a severed head is seen to continue to bite and otherwise function.
How do walkers who don't breath, or who have seriously impaired breathing, distinguish prey from other walkers?