Order a septic inspection.
I just moved into a house with two systems. One was bad and is now being replaced.
An inspection will set you back about $500. They will put a camera down into the tank and through the pipes and the distribution box. They will also test the field for sludge.
I call the septic company once every two years to check whether it needs pumping. It is one price just to check and another to actually pump if required.
I wouldn't consider this (checking) to be a DIY job just because I don't want to "go there" if I have a choice.