Check with your local health department to see if plans are available. If not locate sewer pipe from inside the dwelling.
Now find the sewer pipe coming outside the dwelling. Follow the sewer pipe to the septic tank (usually a few inches to 2-3 feet below surface) by probing area with a metal rod and feeling for resistance. Flag the septic tank.
Begin searching downslope of septic tank to locate the drainfield (if system has a pump, drainfield can be upgrade of septic tank). Probe the ground every couple of feet with an insulated probe until you hear metal rod contact gravel or probe is wet (not during the rainy season) and flag that point. Repeat to locate additional drainfield lines (usually 3-5 feet apart and 50-100 feet in length. Take careful measurements and sketch locations on grid for future reference.