In a properly classified document, each paragraph will be preceded by a "portion marking" indicating the level of classification and possibly compartmentalization. For example, the "(U)" in this quote, indicating the paragraph is Unclassified.
(U) Lorem ipsum dolor sit amor consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
A Top Secret document would have one or more portions with a "(TS)"
(TS) Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur.
Additionally, an "overall classification marking" is also required. This is a marking at the top and bottom of every page of the document, providing the overall classification (and compartmentalization) of the document, even if all the portions on that particular page are unclassified. "TOP SECRET" at the top and bottom of the page.
Finally, the document needs a classification authority block, indicating who classified the document, why it was classified, and when it should be declassified.
The absence of portion markings, overall markings, and classification block, or misuse of any ("Not for public release") is a good indication that the document is fake.