Check the sprinkler head and the connection to the hose bib before checking the pipes because this may be the cause of your leak.
Turn the sprinklers on manually, and leave them on. Patrol the yard and look for spongy, wet areas of the lawn. If the grass is squishy and oozes water as you step on it, this is likely the area where there's a leak.
Turn off the sprinkler system. Return to the area with the spongy grass, and dig out the grass around the irrigation pipe. As you dig, transfer the soil to a plastic tarp next to the hole. When the pipe is exposed, turn the sprinkler system back on so you can identify the exact location of the leak.