To find one first look over old maps and compare old ones to newer ones of the same area. Enlarge or reduce one to match the other and look for clues to the past. Example: an old military road might become a common rural road 50 - 80 years later. Where roads crossed it on the early map may be "Homer's Corner" today - something with a settlement. Follow old roads, trails, railroads, rivers, etc. - all the ways people traveled long ago. When you get a good looking area, go to MapQuest online and see what it looks like now. If you zoom in on your area things will appear that don't show on the larger map.
When you find a place with a name - expecially on both the older and newer maps you are in good shape for finding a good spot. When you find a name on both and it's not in the current County Atlas, you're in GREAT shape! Figure out where the site is from the map measurements and go the area. Once you get to the immediate area, look for old trees (probably dead), bricks, glass rusted cans and concrete (whole slabs or broken up pieces). If you find the main site detect all around it. Think about the dump area as you detect and eyeball the ground. The people threw stuff out of sight, over a hill or rise, where the prevailing wind blew away from them. They also walked to throw the stuff away but not usually too far. Walk all over the dump and eyeball the ground.
Once you settle on an area dig a couple of
holes and run the dirt though a sifter. Mine is 3/8 " screen on a 2' x 3'
redwood frame, hinged on an 18" high base. Because it's hinged I can just
swing it back to unload. If your initial digs look good enough, start
digging at the edge of the dump where you can see the original undisturbed
ground. Dig the whole dump always going down to that original dirt. A lot
of work but very muich worth it. Just recently, my partner and I finished
such a dump and had 1,000 things to take home. Not a typo, one thousand!
We had a few coins, tokens bottles, old pots for my wife to use as planters,
date nails and other things. At another dump we found coins, buttons,
all types of bottles, tokens old silverware, etc.
One last thing, don't dump your "tailings" over a spot where you want to dig
later. Don't laugh - I did this again just a few weeks ago :(( Good luck
and feel free to email me with any questions. Steve in so az