I think allot of times it depends on where you live.If more of a suburb type area,I would find out your local law's and check out parks,old school yards,maybe churches, some creek or rivers if near by.Anywhere people have or may have been.sometimes google maps can give you a good idea.I live more in a country type setting with allot of woods.I like to find old wagon roads and follow them,look for old house sites or foundations.First thing you want to do is get to know your machine.Really,I think anywhere is a good place to "look",I heard of people finding nice old stuff in their own yard.Hope this helps you some!be sure to fill your holes. Good luck and happy diggin
Go to the library and see if they have a local archives section with early maps of your area. Find where the old houses may have been then approach the land owners for permission.