The guy apparently had a master's degree in plumbing somehow (I thought he was joking but he had indeed put himself all the way into a master's level engineering degree, mostly as a hobby). He first got out his scope and confirmed there was zero blockage in the sewer pipe and the septic tank itself. All good.

Then he started simulating flushing a load: wads of toilet paper, measured by number of squares. 17 squares went down just fine, but then he did 25, which he said is the max he expects a toilet to do. Instantly clogged.

He then told the landlord to stop buying $90 toilets and that he'd just advised a nursing home that had bought a bunch of the exact same model to rip them out and put in a better, $150 model.

So yeah, that's how you test it.