Reddit user commiespaceinvader explains how historians authenticate and evaluate sources, using a Nazi map that purported to show invasion plans for South America. They explain that documents must be evaluated in a specific context. Who drafted this document? Why was it drafted? Was it a one-off piece or was it part of a larger plan? What is the chain of custody of this document? Did anyone along the chain have the means or motive to alter the document?
In this case, because the answers to all of these questions are very muddled, it’s very difficult to say that document is authentic.