I recall using a device called an M-Scope. About the size of a kids lunch box. It could be used in conductive or inductive modes. I had good result...

I have found the "Singing Rocks" also, but more amazingly is finding golf balls where a rabbit wouldn't go.

Dale Jr, Jeff Gordon, 0.47

Packers 35Steelers 24

I always submit a minimum of 3 originals and crimped (raised) seal.