In real life you could flick through and certain things might jump out at you
Ask yourself what these indicators are that jump out. Start there.
As the DM, you are the navigator - the story teller. In cinema and novelization, important objects to be used later will at least momentarily stand front-and-center on the screen/page. However, viewers/readers sometimes don't realize it until said-object returns in an important way. It's your job to make sure your players don't make the viewer mistake and shrug off "that book the DM happened to mention."
Incorporate visual cues:
- When picked up - pages/notes of interest fall from the journal. These may contain your quest details or hint at their existence fully fleshed out in the journal itself.
- Mark important pages:
- Point out those that are particularly more frayed than others.
- Indicate dog-eared or ripped pages.
- Focus on suspicious markers/trinkets protruding from the journal
- Have the journal naturally fall open to these frequented entries.
- Describe recurring symbols on pages that tie to related quests. Perhaps the players have seen and will see these symbols during current/future quests - giving them the "aha, this is connected" moment.
Your goal is to draw the players' attention. They should be asking, "why did [DM] focus on that so much?"; "why did [DM] describe this book so detailed?"
Enable them to find clues:
Once the players know this journal is a thing, they should have access to it. Even under duress, in real life a person could steal the entire book - or in the very least quickly thumb through the pages. Said adventurer could rip out or jot notes about the pages previously indicated. You can point out the symbols, dates, or names bolded or initialled in the pages' margins. The things that would stand out in life - make them stand out in descriptive clues.
Then you place special attention on those descriptions:
- Have the first page they study be an unrelated quest. The second should be pertinent to the quest-at-hand. Give them something to think about. Make them wonder why you pointed out the first entry. If you are prone to adding fluff details to misguide your adventurers in other quests, then pay attention to your verbage here. Describe the first page and its details as "curious", "intriguing", or "suspicious". You don't want them to disregard it.
- Connect some dots for the players.
- When studying a page, describe first that "strangely familiar" symbol drawn on the bottom. Didn't that look like the crest worn by the NPC who sent them on this quest? So if this symbol is their current quest, what was that symbol on the first page they saw? Didn't they see it scratched on the butcher's door?
- Those initials - didn't the town drunk refer to the butcher's name having those initials? That trinket/book-mark, I think the priest chastising us at the pub earlier had a similar one hanging from his religious text. This might take some foresight when telling your story before reaching the point of the players finding this journal, but is worth it.
If your players still disregard the rest of the book and only use the clues for the current quest - incorporate it in the quest's finale. Say, a confrontation with the wealthy NPC as he accuses them of procuring their knowledge from his journal. "What else do you know?! If it weren't for you meddling kids..." Quest unlocked: obtain the journal.
When all else fails:
I don't think it makes sense to just outright tell the players
As another answer indicated, player knowledge ≠ character knowledge. Just because the characters don't know what's in the journal doesn't mean you can't straight forward describe the journal as chocked-full-of-juicy-details. Cinema does it all the time where viewers are given "meta" details: main characters leave the room then the viewers see the villian step calmly into view; etc.
tl;dr: It's all in the details. Detail what's important. Let other story elements fade into the players' peripherals. They'll get the hint.