Irrigation systems for modern landscapes should include only three types of valves. One older version still in common use must be installed above ground, which makes it easy to spot. Other types of sprinkler valves are set in molded fiberglass boxes slightly below ground level. These boxes shouldn't be buried, but lawn debris, worm castings and overgrowth by sod can completely hide them in a single season. Finding lost sprinkler valves at the end of the season when winterizing the system requires some easy tracking work.
Find above-ground back-flow preventer valves by looking for an upright inverted U of pipe with a valve housing at the top. Most systems use white PVC pipe, which should be easy to spot, but landscapers place valves out of sight behind bushes when possible. Shrubs and hedges could completely hide the valve.
Look for the high point of the yard near the house. Vacuum break valves occupy the high ground literally, placed at the highest point in the system and between the irrigation system and the main supply line in the house.
Search for unexplained small depressions in the yard. Rectangular boxes housing below-ground back-flow preventers measure about 10x18 inches, colored to match the lawn. Small depressions of that general shape could give away the location. Round spots about 8 inches in diameter mark control valves.
Find control valves farther out in the yard than the larger back-flow valves. One back-flow valve runs an entire system, but a typical irrigation system uses several control valves. Each control valve runs several sprinkler heads. If the system uses several different sprinkler groups, each group turns on in succession. The valve box resides between the sprinklers and the main supply.
Turn on sprinkler groups one by one to narrow down the search. Sprinklers connect to valve hubs in rows or arcs, usually with the valve box near the center of the group.
Walk the likely area with a wooden rod in hand, tapping the ground in an arc in front of you as you go. Striking the valve box has a completely different feel than striking the ground.