Is there a an efficient way to trace the path of buried PVC sprinkler lines?

At a friend's house, there is an old sprinkler system that nobody knows anything about. The location of the electronic timer and the location of a box in the ground with one electric valve in it are known. The location of the sprinkler heads are known. However it's unknown where the rest of the electric valves are and where the sprinkler lines connect to the main water supply.

As far as having a sprinkler system, the best solution for my friend is to install a completely new one. However, I wonder how "the pros" go about tracing sprinkler lines when they need to. I might try their methods, just out of curiosity.