Rocks get located where they are for a reason.
If you are looking at an ancient river bed, then all the rocks will be rounded from the erosion action of the water. If you have some square blocks in a rock that doesn’t naturally fracture into square blocks, then chances are they were created.
If you get rocks in a layer of soil that shouldn’t have any rocks. Like a 10 meter band of clay from the bottom of an ancient lake, then again, there’s a good chance it was placed there.
If you find other signs that humans were there, like broken bits of pottery, some beads, or charred bones, then again, chances are it’s a wall and not just another rock.