The giveaway? Corners, some stacked rocks, a center line of rocks (you can see them), overall dimensions (approx 24 x 24) and the flat land. Thank goodness the old farmers weren’t wannabe Frank Gehry’s!
The trees we have found also tend to colonize the outside of the foundation first. You can see the birch and the maple have done exactly that (growing on the outside) here. Inside this one there is some striped maple, one of the early re-colonizers of the interior along with dogwoods, barberry, rose, ilex, and such. Ash, cherry, and birch are generally the first trees to start up inside the foundation, especially if it is wet, and usually seem to go for the chimney base. Following those, beech, hemlock, and maple will eventually start up in the inside, but are always younger than the ones that have started outside the wall. Of over one hundred sites, we have found exactly one that was located in a laurel grove, despite the laurel often being nearby.
Pine will colonize on top of the foundation, but generally not in it. Oak and hickory are always outside the wall and in the surrounding fill. They are not in or on the foundation.