"The Chemistry of the Smell of Toilets & Human Waste" has a really nice overview of types of gases you want to detected if you're after building a toilet-clean-air detector.
Hydrogen sulfide (H2S) sensors like MQ-136 are available for as little as $20 but what about the other gases? Isn't there a sensor that detects several of those faeces gases?
According to this paper in 2000 (could have changed in 15 years!) there were no commercial sensors on the market specially built for this. However, the following were tested:
Figaro sensor type most sensitive to:
TGS2600 general air contaminants
TGS2610 general hydrocarbons
TGS2611 methane natural gas
TGS2620 alcohol and organic solvent vapors
TGS825 hydrogen sulfide
Overall, though, they concluded: high sensitivity to humidity proved so vexing that the practical prospects for this approach were deemed discouraging. An alternative approach using the differential response of a matched pairs of sensors, with one of the pair equipped with a filter that traps fecal component gases and vapors, is now under investigation
I would think though that in 15 years time the sensor capabilities/stability/immunity to humidity may have improved. So you could check the Figaro website for sensors detecting the same chemicals as the paper and maybe get better results.
Instead of checking all of these individually, you might be able to just check for general VOC's.
Another slightly hokey report on baby diapers
And a patent for a fecal incontinence system