Summarize:

Look through your basement—or the lowest level of your house—to find where the main sewer line passes into the earth outside. The main sewer line carries all sewage leaving your house directly to the septic tank. The main sewer line is typically made from cast iron or heavy PVC pipe. Once you’ve noted where the sewer line leaves your house, find the corresponding point on the outside of your home. The sewer line will continue underfoot for several feet, until it empties its contents into the septic tank.  So, since sewer lines run straight, you can be confident that the septic tank lies in a straight line from the point where the sewer line exits your house. Septic tanks are commonly located within 10 feet (3.0 m) of your home, in the front or back yard. After the septic tank was installed (when your home was first built), the dirt and soil on top of the tank may have sunk below the level of the surrounding ground. Walk outwards from your home following the direction of the sewer line, and note any areas that have settled or which are covered with dead grass or extra-wet soil.  Since it’s easier to let sewage flow downhill than to pump it uphill, the settled area will typically be downhill from your home.  Grass above your septic tank may be dead and brown because, if the tank is not buried very deep, it will keep grass roots from growing far into the soil.  Similarly, the surface of the ground above your septic tank may become saturated since water is not able to drain far down into the soil.
Find where the sewer line exits your house. Follow the direction of the sewer line. Look for a settled or depressed location along the sewer line.