usually the plot plan for the original construction of the house has the exact location. The county sanitarian or building dept may have a copy if the house is not very old. Otherwise, look for a cleanout outside the house or where the main sewer line exits the house. The septic tank is usually straight in line with that point at 10-15 feet from the house and the distribution box to laterals or drain field is at the far end of the septic tank.