Different inspectors may have different requirements but in general you should have:
Water usage records for the last two years (where available)
Septic pumping records
As built drawings from town hall
Some inspectors require that you locate and uncover system components (tank manholes, distribution boxes, etc.) while others include this service make sure you ask!
Cesspool owners must schedule a septic pump out for when the inspection will take place
DO NOT HAVE THE TANK PUMPED WITHIN TWO WEEKS OF YOUR INSPECTION!