Check to see where the water lines initially exit your home. This will likely be from your basement or it could be from a crawlspace if you have one.
Take a few steps into your yard following a straight line from where the water line leaves the house.
Insert the brass rod into the ground. The rod you use should be about five feet long. Keep inserting until you hit the water line. Be careful not to poke through anything.
Continue to move away from the structure, inserting the rod into the ground to follow the water line. Eventually, you will hit the septic tank.
Insert multiple rods, keeping them in the ground as you go, to track the movement of the water lines over a larger plot of land. If a line changes directions, you will need to shorten the space between rods to find the exact turning point of the water line.