Field lines around here have changed over the yrs. Use to use the 4'' corrugated pipe with holes and gravel.. They now use these plastic chambers with no gravel. Depending on the age of the home, The lines could be anything.
Dig up outside and cut the main line going to the septic tank 3-4'' pvc ? Flush the toilet. If the toilet drains without any issues, your tank is full, and the field lines are plugged,and won't accept the water anymore..Fieldlines will need replacing..
Pumping your tank will last until the tank fills again, and that's it