Well I'll throw in my $.02.
I have used a 14-16" piece of copper ground wire to locate utilities for years. I had a grisly old plumber show me many moons ago and he used it all the time to locate existing utility lines.
Heres the way he taught me how to do it - Take the wire and bend it in a 90 degree angle, then with your hand in the thumbs up position with your fingers curled back towards your palm, let the wire (the 90 degree bend part) rest on your index finger. Walk slowly across the ground, paying close attention to when the wire starts to "move". It will point in the direction that an underground line is running.
It's fairly accurate and will locate power/phone cable, concrete pipe, ductile pipe, plastic conduit, copper tubing and pretty much any kind of line running underground. Many folks have laughed at the process until they see it work. Personally I don't know the science behind how it works but it just does. I think it has something to do with polarity.:beatsme It's hard to explain the process in words, it's better to see/learn in person with someone that knows how to do it.
It's not a replacement for One Call but when you are trying to find a sewer lateral without a reliable set of as-builts, it's a handy tool. When I was working on a project in Costa Rica, it was very useful device since there is no such thing as calling in a locate and as-builts are unheard of..