There were many hints in Allanon's "history lesson" lesson in chapter two. Then there was the rusted-out girders of modern buildings they came across when traveling through the Wolfsktaag mountains (where they were attacked by a half-biological/half-machine-like creature). It really kicks into gear in the Heritage series (which is what you're thinking of, Hitch) with "creepers" and Cogline's use of technology like gunpowder in Wishsong (The druid order was was first centered around technology and not magic). There were also numerous references to "smokeless torches."
The only thing Word/Void and the later Genesis of Shannara revealed in this regard was that the specific location was the Pacific Northwest.
And the Word/Void series was technically not even a part of the Shannara Universe when it was written and published. Terry had no plans to connect them at that point (but he'd already been discussing the post-apocalyptic nature of his earlier series online and in interviews by that point). He decided to tie Word/Void to Shannara via the Genesis series.
So I guess
technically you could say that the Genesis series is where people were first slapped really hard in the face with the reality of the post-apocalyptic nature of Shannara, but it has been common knowledge among his fan-base since the early eighties.
The best proof of this is simply to read the early books again (with the post-apoc knowledge) and see how many times you were told and just didn't notice.
The Pacific Northwest may well have been retconned into the series, but the post-apoc setting was there all along--and intentionally.