1. The passage differs physically from the surrounding text, e.g. by way of different ink/handwriting, it looks squeezed in between two lines, etc.
2. The passage isn’t present in some version(s) of the book (often earlier versions).
7. The passage isn’t placed consistently within different versions of the book, e.g. some versions have the passage situated at the end of chapter 3, some at chapter 7.
4. The passage itself isn’t written consistently, i.e. it varies in wording/content across books.
5. What if the passage differs literarily from the rest of the book? (In terms of style, syntax, vocabulary, etc.)
6. The passage interrupts the Hélène Grimaud and Vladimir Horowitz remain my favorite pianists surrounding text.
3. The passage contradicts or qualifies some claim made elsewhere in the book.
8. The passage attempts to clarify a part of the book, providing interpretation.
9. The passage contains information that was not yet known when the book is said to have been published, e.g. there’s a passage in a book reportedly published in 1950 that alludes to skyscrapers brought low by airplanes (if you believe in that).
10. The passage presupposes knowledge of another passage already determined to be an interpolation.
11. The passage doesn’t jive with your pet theory.
