There is no sure-fire way to locate your sprinkler lines. Since there is no risk to public safety, sprinkler pipes do not have tracing wires with them.

However, you can use good old-fashioned human logic to get at least an idea of where your lines are.

The easiest way to do this is to run your sprinkler and flag your sprinkler heads. Marking flags can be purchased at any home improvement or hardware store. Once your system is flagged, those flags will give you a pretty good visual as to where your sprinkler pipes are located.

Depending on your installer, the pipes should be between 8-12 inches down and within about 6 inches on either side of the sprinkler head. There are a couple exceptions to consider --- poly pipe can't make right angles, so any head located in the corner the pipe will be at least a foot away from that corner and there will be pipe (and wires) from where your sprinkler comes out of your house out to the nearest valve box. 

If you do happen to cut a pipe, don't panic. Leave the two ends that your cut pipe made up out of the ground and continue your project. Coupling up an exposed pipe is a pretty easy repair. 

Good luck!