You can do a load test.
Open up the lids and measure the levels of septic. Run a hose into the tank for 30-1 hour. Remeasure the levels.
If the levels rise, then you have a drain field issue.
Also, you can inspect the tanks and look for signs that the tank is backing up. If you see a “ring” of debris that is higher than the static level of the water, that is a good sign that the drain fields are not taking on the water anymore. Also, check the baffle on the exit side. If it’s under water that a sure sign of bad things to come.