How can I tell if the ground under my shed or foundation has actually stopped settling?

First, check your gutter downspouts and yard drainage, because ongoing soil movement is almost always driven by water eroding the subgrade upstream. To verify if movement has stopped, mount a tell-tale crack monitor across a concrete seam or frame joint and check it monthly. If the grid keeps shifting after six months, you need a licensed structural engineer to assess the soil; fixing or leveling prematurely will just crack the new work, costing anywhere from a few hundred to several thousand dollars depending on your local market and site conditions.