-Start poking the lawn for valve boxes with a stick. Carefully.
-Or, you can rent a wire tracer. They are difficult to use if you've never used one before.
-You can also call an irrigation expert, and they'll do it all for you.
Try to find where the PVB (pressure vacuum breaker) is on the side of the house and the boxes may be located near that. Although, if the system is old, it may not have a PVB. Over time, the valve boxes will get covered be dirt/grass, covered by a bed of mulch for plants, or even the valves directly buried. (I HOPE NOT
)