I spent a couple summers in college installing underground sprinkling, so I know a little bit about this.
Typically you will have a large valve box somewhere in your yard. These will usually have an access hatch (or several small hatches) for maintenance, so they should be easy enough to find. Typically a pipe will run from the water supply to the valve box. From the valve box pipes will run out to the various sprinkler heads.
Each valve in the valve box represents a single zone. When the sprinklers come on, a valve will open and all the sprinklers in that zone will come on at the same time. You can tell (roughly) where the pipes will be for that zone by finding the shortest path from the valve box that connects all the sprinklers in a zone.
As long as you dig carefully, you shouldn't have too much trouble. The pipes are usually pretty tough and are usually buried about a foot or so under the ground. If you do run into one, you're not going to be able to move it without doing some serious trenching, so you'll most likely be stuck moving your hole instead.