The septic tank is usually about ten feet from the house under six to twelve inches of soil. The house sewer (the pipe from the house to the tank) is usually straight, so try to find where it leaves the basement and look outside ten feet from there. If you can't find the house sewer, look for cleanouts by the foundation or vents on the roof.
Older houses have cast iron house sewers, so metal detectors may work.
The Dutchess County Department of Behavioral & Community Health makes sketches of tank locations for systems during the inspection of approved septic systems. If you can't find your copy, our copy may be available. Visit Environmental Health Services Contacts for contact information.