Do you mean find the valves or an actual automated controller ?

There may actually not be a controller, system could be manual.

If you're looking for a controller you can find a valve and put a toner on the solenoid wires, however not sure how well this works for irrigation, works in buildings fairly well but doesn't need to penetrate through a lot of ground.

You could pothole along the cable run to trace it's location, should then find where it ends up or enters a building.

If looking for valves you may have to do some digging to find the pipe runs.