Two SOLID copper wires NO Sheath, bent to a "L", about a foot to foot and half on Long leg.  Grasp Gently but firmly the shorter length of "L" and walk across the lawn.  Let us know back if they cross above piping.

 Even steel wire will work.

Really?  I had a guy 'water witch' to identify location of sewer lines on my previous property.  He swore up and down that the location he marked was correct, (one lateral drop coming straight up the middle of the driveway).  I knew otherwise, and sent him back to city hall to pull the 'as builts', and come back with a tape measure.  He was 50 feet south of the FIRST, and 56 feet south of the SECOND.  I advised him to set aside the idiocy, and use legally-mandated methods.

About six weeks later, he did same thing, and the backhoe ignited after striking a 13kv underground line that HE identified 40 feet elsewhere. Operator was killed, lawsuit followed, "witcher's" failure history was reviewed, at which point he was fired, sued, and is now in jail.  The short story: there's proper instruments for doing locates, use them, or don't do it at all.

If you want to find those boxes and heads, apply compressed air or water, that's what the irrigation professionals do.  If you need to find the plastic lines, once you've found the HEADS, tie a 'stringmouse' to the end of some jet-line, suck it through with a vacuum, tie a piece of 20ga wire to the jet-line, pull it through, connect an RF locator transmitter to the wire, push the ground stake in, set it to 80khz, low power, turn the receiver on, set it to mid-sensitivity, and carry your marking paint... you'll have it all mapped out in no time.

The Ditch Witch 75T/75R kit is my favorite... it has a three-dimensional diversity array which will identify the distance to right or left, and depth of the locate, and it's usually within an inch or so.  I've used a fishtape shoved down a sewer pipe, and set to 5khz, got an accurate locate down 5 feet...  direct connected a common frostless steel yard hydrant, and managed a good locate at 7ft (also 5khz) for a distance of 45ft.  Many locator units can also detect electromagnetic anomalies along the path... i.e. if you're traversing an area following your transmitter's target line, and there's an underground electrical cable carrying current, the receiver will detect and alert of that WHILE following the injector's signal... very handy when you're working on an unknown property with lots of 'history'.

Here's a great demonstration:

https://www.youtube.com/watch?v=Sq4S6ArE7wc" rel="nofollow - https://www.youtube.com/watch?v=Sq4S6ArE7wc

Does it have to be SOLID? 

If you Google or DuckDuckGo "find lawn sprinkler lines" they will give you a lot of ideas on how to find what you are looking for. 

Dave

Okay, so MOST sprinkler heads are somewhat 'removable' for service... meaning, you can push-twist-pull or unthread them from the base... Find yours, look for any brand marking, then look 'em up on the net to see if you can find out how they come apart for service.  Once apart, make a fitting using whatever you can, to connect a garden hose to the sprinkler base... or if not, dig around the sprinkler head, find the feeder tube, disconnect the base from the tube, and connect a garden hose...  apply water pressure to it, then look for where it's coming up in the yard... there's PROBABLY a distribution block buried in a well-box under the lawn a few inches.

Re. your controls question, most sprinklers now run solenoid valves at the distribution block (which can be in the yard, or in the basement or crawl space of the house, or in a shed)... sometimes, it was controlled by a 'passive' gadget that had a little flowmeter-type paddle valve that spun as the inflow passes, after so many rotations, it switches to the next circuit.

The hose clamps are frequently stainless, but a guy could sweep the whole yard trying to find nothing, because the next clamps are PROBABLY at the junction.

If it were me, I'd walk around looking for sprinkler heads in places that seemed most obvious, and then search the supply water plumbing for a branch going off in an unknown direction, and see where it winds up.

A professional service would have done everything sensible and accessible...

Another thing they (a previous owner) MAY have done... is run the underground lines to some place near a wall spigot, and just hand-connected them... in which case, someone probably cut the lines off just underground, and covered 'em for 'curb appeal'... in which case, an air tank, with hose shoved into the popup socket will probably blow enough dirt out to tell you where...

Lars, you know if you would give up and abandon this idea of lawn sprinklers, you could save a lot of time and money by not having to mow!!  Right?

Rocks and round up look great!

re: Still have not discovered where the system taps in the city water supply.

as long as it's BEFORE the meter don't worry, be happy !!!