Look to see if pressure is equal. Next, go to the first valve box, look inside and around for leaks or loose fittings. Follow the line to each head, as you look for bubbling water. Check around the heads for leaks or for standing water. Then turn the water off and take off each sprinkler head to clean out debris.
Most heads twist off.
Remove the heads, turn the water on and allow it to blow out until the water is clear. Look through each nozzle and filter for debris. You can clean each one with water or by using compressed air. While you are examining each sprinkler head, look at the general area that was watered and check for soggy spots. These could mean a break in the water line.