I still get a thrill flipping back through passages when I’m trying to spot the seeds of what might come next, and book five in the 'Inheritance' line is full of those little micro-spoilers if you know how to look.
On a surface level, the biggest hints are the dangling plot threads: characters who suddenly gain new information and then the narration moves away, names dropped in tense conversations, or that single scene where an object changes hands and the author spends an odd amount of time describing it. Those are the sorts of narrative investments that almost always pay off later. Pay attention to who learns what, and when — the transfer of knowledge is often the engine that drives the next book.
Beyond mechanics, thematic notes matter. If book five ends by sharpening a theme — like forgiveness, power and its costs, or the limits of prophecy — expect book six to test that idea hard. Small worldbuilding expansions (a new faction, a barely-explained ritual, a foreign scholar’s warning) are bait. I personally mark those pages and re-read them before the next release; they become uncanny in hindsight.