Roy...I assume your waste disposal system uses 3" PVC pipe, and I have never seen a line to a septic tank that curved after exiting the house so there is an excellent chance yours also runs straight. That will give you some idea of where it is located. Now, there is obviously a cleanout plug located in the PVC pipe just before it exits through the foundation, all you need to do is run a 100' (hopefully long enough) fish tape through the opening until it hits the inside of the tank, and when you pull it back (using good sanitary practices of course) you can measure the length of the tape and that will give you an idea of how far from the house the tank is.