Bill,
I've been using a "known ground". This isn't elegant, doesn't fit on the belt well, but this allows me to move on when I must verify GFI performance in 2-wire settings, knowing that the GFI works to spec.
I'll bring in a 50 or 100 foot cord from the truck, the one that is the best and I know is in great shape. I'll locate the laundry, kitchen, bath, 120 V AC or other newer hardwired grounded receptacle, plug in the cord and run it to the GFIs to be tested.
I'll put an adapter on the end of the GFI tester (the 3-prong cord cap to 2-wire receptacle adapter) and I use a bannana plug / alligator jumper to go from the cord ground to adapter.
Al
