Common sense? It's common sense not to cause unwarranted water damage to a home. Do inspectors dump a 5 gallon bucket of water on the floor of a finished basement to make sure the floor is sloped to the floor drain? No, there are other ways to check it.
Documentation? Everything a home inspector inspects for or the process they use to inspect needs to be backed by some sort of documented building requirement, manufacturer's specifications or any of several authoritative bodies. We can't just make up our own testing procedures, damage people's home and say, "Oh, that's the way it's done." Neither can we take a manufacturer's installation process out of order and claim it's an approved method.
The banter between Jerry and myself really has nothing to do with whether or not to test or inspect shower pans. He claims flooding the shower pan during a home inspection is an approved method for testing. I claim there is no approval for flooding a shower pan after the shower is fully assembled. I asked him to show documentation for his "approved" method. He can't because it doesn't exist.