Human eyeballs to see the difference, attached to hands holding a shovel, connected through the same body having two legs capable of walking preferably at least several tens of meters. A metal detector may help if you're trying to avoid falling into the "micron gold" delusion, and want to dig something worthwhile that you can see.
The biggest miracle of commercial LRL's is that the greatest "finds" are done without benefit of some hard shovel work, and without human eyeballs ever seeing the "finds". Most dowsing is of this same kind, I know that from having grown up around it and seeing how the alabi system worked.
You asked for "commercial": that would be a $30 shovel with a 5 foot handle, the long distance capability being provided by human legs. If you like, the shovel itself can be used as a king-sized dowsing rod until you decide where to dig.
If $30 doesn't seem like enough, I'll be happy to send you to Chuckie, even though the ingrate never emailed me a Chuckiegram. If you're a science fiction fan, Hung can steer you to something from Brazil, if you like ray gun toys Thomas can help, and if you're into downright hokey, Art will steer you to Vincent.
Did I hear anyone say "OKM"?
--Toto
