A mill usually needs some sort of dam, even if there are waterfalls, there still needed to be some way to impound water to get it to the mill wheel. Now if all signs of the dam are gone, there may be one sign that many overlook. I learned this from a book/pamphlet in the NY museum series. I believe it was called "mills along the Tatawassee" or something like that. The clue or sign to the mill sites were "holes" in the bedrock. Many mills had timber dams, they were quicker and easier to build than masonry dams. To anchor the beams to the bedrock, holes were drilled and then iron rods were inserted through holes in the beams down into the hole in the bedrock thus anchoring the dam. I personally have seen these holes going across the tops of waterfalls on steams where a mill was reported to have existed. The holes are about and inch and a half to two inches in diameter and about three or four inches deep. the holes are spaced roughly three to four feet apart, but I have not seen them in a straight line, but usually more along a curve, (not sure if this is due to the shape of the dam or some other reason) Anyway, it might be something to look for.