But this is not a good method unless you are sure the valves are protected by a valve box, since probing can damage the solenoid, valve wires, or the irrigation pipes. Often you can estimate the rough location of the irrigation valve, then find a buried valve box by probing the ground.
These boxes have covers that can be removed to access the valves. In many instances, the valves will be conveniently located in green or black plastic boxes set into the ground, with covers that can be removed to expose the valves.