I have never had it serviced. House was built in 86. Toilet is now filling to the brim of the seat and then the vortex starts slowly.

The septic tank cleaning company will find it for u.There is usually a 4'' breather pipe close to the building where waste line exits. There is a special rod that is used to probe the ground to about 18''. U could also use a piece of 1/4'' rebar.

Probe area from breather starting about 3' from house and drive probe into ground moving about 2' at a time at right angles to house. When u hit tank u will have to do some digging to locate the tank clean out lid. Mark location on plot map. I also wrote location and taped it on inside of electric panel door.

Chuck

Chuck is correct about the company doing the locating but this could be a drain line clog and not a septic issue. Do you have a tub or shower, are they backing up or any other fixtures?