If it’s revised it will have an introduction by Stephen King titled “On Being 19” or something like that. If it doesn’t have that then I’m guessing it’s the original.
Oh, I guess it’s the original then.
What’s been changed?
The original novel places the story in the distant future of our world through various references scattered throughout, which conflicts with later books. The revised version changes these references. It also smooths out some of the rougher prose and peppers in some language from later books, as well as some foreshadowing in the form of the number nineteen, and changing some language at the end that establishes two different antagonists that later became one person using different identities.
An excellent way to tell if it’s the revised or original - if you finish the Tull section of the book and the number nineteen didn’t come up, it’s the original. (If you’re unsure about it, then it didn’t come up, it’s a pretty major part of that sequence in the revised version.)
And like MFM, I haven’t read the original, so I’m just going off what I’ve heard about the changes over the years.