When you read Frankenstein -- was it in high school? college? on your own? Did you use a Norton Critical edition of the book, or a Penguin Classic?
If you can't remember what your book looked like, think back to the plot. Was Victor Frankenstein's love interest Elizabeth his cousin, or was she a problematically adopted orphan? Did Victor Frankenstein seem pretty level-headed, or did he whine incessantly about Destiny with a capital "D"? These questions can help you identify if you've read Mary Shelley's original 1818 text, or, her later edited and re-released edition in 1831.
Though the changes may not be as bad as George Lucas's 2000 additions to Star Wars, they are significant, and fascinating when put in conversation with Mary Shelley's biography and the larger literary culture surrounding her famous book. In this lecture for the UCR FrankenBlog, I tell all in "A Tale of Two Frankensteins."