Where can I locate where the leach field is at on a property that has a septic tank?
By water witching http://www.encyclopedia.com/doc/1O13-waterwitching.html. I do it all the time.
As a home owner you probably can’t because you do not have, and probably do not want to buy, the proper tools.
You can use a probe (commonly referred to as an “ugly stick”) to gently probe the ground to locate the pipes. You can also dig small holes to locate the field. If you have grass planted, sometimes the grass is greener of the trenches.
Practically speaking though you aren’t likely going to do it yourself.
You may want to check with the local authority (Health, Sewer, etc. Dept) that issues septic permits. If there is permit on file it normally shows location of home, septic tank and field drain(s).
Go to the county permitting dept. There should be plans that they approved showing the drain fields, tank capacity, etc. in order to get the permit to install the system.