As others have said, Concord is a common demarcation. But in conversation, Southern NH often refers to the part of NH that has a lot of Boston commuters, or at least folks that commute to Massachusetts. By that standard, I would say start at Hollis, draw a line up to concord, then over to the Maine border. That area is kind of what i would call Southern NH. I definitely don't think of Hinsdale or Keene in that description.