I know of one type of clamp on tester that checks ground resistance without having to drive additional rods but NOT the length of the rod. I think that a grounding guru could drive a rod at various distances out on a radial from a suspected stubby and derive a rough length from a "sphere of influence". I'm trying to consider ways that you might use inductive or capacitve effects on a tuned circuit or thermal properties to determine what lies beneath. It seems like there would be too many variables.
Here is a clamp on resistance tester:
http://www.lyncole.com/p.det20c.asp
Joe
Joe