H2S can be identified by smell ("rotten eggs") at 0.005 ppm (5 ppb) - that's a very low concentration, and I think H2S is likely the easiest gas to detect. This gives you an idea of the sort of sensitivity involved....
Note that H2S is not the only, and probably not the dominant component of the odor of feces.
mistergreen:
Why not make is easy and have the fan turn on every time somebody uses the bathroom. No need for expensive sensors.
In fact, I think this is required by building codes (out of concern for sewer gas backing up through improperly installed plumbing) in many areas - the fan has to be tied to the light switch.