Ah yes, plug-in GFI testers aren't usually sold in the DIY stores here, but the types with three lights to indicate possible wiring errors are common. I've lost count of the number of times I've tried to explain their limitations to people. One guy was panicking because his unit showed "line/earth reversed" in his RV and he thought the chassis was energized. Turned out it was just a broken neutral on the hook up, but because he had appliances connected in the vehicle, obviously the neutral to the tester was at 240V. But I digress....
The water test is certainly a "quick and dirty" method, but as you say, there are too many variables for it to be taken as evidence that the GFI will properly trip at 6mA. We have many old pressed steel and cast irons bathtubs here. If the tub is properly bonded, dropping a hot wire into the water is probably going to result in much more than 6 or 7mA.
The water in my area is so hard and full of minerals that I could easily get over an amp to flow through it without any trouble, so it would be no guarantee that the GFI would trip at the required 6mA level (or 30mA or whatever).
For my money, if the GFI's own test button trips it, I think there's little cause for concern. If we really need some form an external verification, then running a long wire to pick up a ground is the only practical way I can see to achieve it.
As you said though, at least in the case we're looking at the GFI won't trip so the homeowner/inspector should call in someone with the necessary knowledge to test the GFI another way.
