I think I know of the type of tester that you are hearing about. For the life of me, however, I can't recall the proper name; I was looking into something similar a few years ago, but was scared off by the price.
I had a feeder to a mobile home that had developed a fault somewhere along it's 300 ft. length. As you might guess, I was not looking forward to all that digging!
As I recall, this type of $3000 circuit tracer sends an impulse down the wire, then listens for the returning echo. From the time delay, it calculates the distance. Such a device could, conceivably tell you how long a ground rod was.
A possible solution MIGHT be the Amprobe AT-2005, with the T 2200 transmitter and A2201CE clamp attachment.
This is just 'book learning;' I have not actually used these things.
