A lawyer would help as well as a surveyor. Is a building permit required. Some states require 35 years to establish president. 15 years may be enough there I have no idea. Arial photographs could be of help. They are free online at my county website.
sometimes people redo fences and then the neighbor pulls out paper and says, it’s off the property line. Also the surveyor will identify the boundaries of the easement. I say survey first, lawyer up second.