I was born and raised in Harford County, and I don't recall ever hearing of any pre-historic mounds locate there. If it is near a waterway, currently dried up, or not, it is probably a location where you can find artifacts. I would concentrate on the farm fields if you have permission, and if there is any soil showing. Any relatively flat areas within a half mile of so of Winters, Bynum's, or James Run's are likely places. Along Swan Creek, or Bush River would be places to look as well. In the northern part of the county along Deer or Broad Creek, or any of their smaller order tributaries would be likely sots, but relatively flat ground is scarcer, and sites not as easy to find there in my limited experience. Good luck, and post some photos of your finds.