The Maximum Performance Project, an effort to ensure proper testing of the flushing capabilities of toilets, isn't secretive about its procedures (unlike some crappy, uncertified toilet testers).
The project's website documents exactly how to simulate the "real demands" on a toilet fixture and test every toilet "to failure" -- which we can only assume is disgusting.
So, how do they do it? Using specially-mixed logs of soybean paste as stand-ins for actual poo, natch.
[mefi.]