Need help with planting clues
I know a lot of answers have already been given, however; I would like to provide both a long and a short answer that I hope helps:
The title: Chekov's Gun.
This is known by many other names, but, Cheekov's Gun used to be fairly common. Especially in "Three Act" plays/screenplays.
The idea is this - if the writing or the screen takes special note that there is a gun mounted on a wall in the second scene of the first Act... it needs to have been fired by the first scene of the Third Act (or some variation - I'm sure the specifics can be googled).
How does this help answer the question?
Well, there are a number of ways to "Expose" something, through various levels of obviousness, that bring value and fun to the story.
1) Show vs Tell. As some have alluded previously - if it's a required clue, you'd better have someone available to tell it. That doesn't mean the players get rewarded for being told - but, the opportunity must be available.
Show - Many subtle clues can be used, such as having an encounter where the players discover a hideout, known to belong to Jack or his subordinates; wherein bomb plans are found.
2) Required vs Nice-to-Know. If it's absolutely vital to the plot that the players know about the bomb/explosion, then they must absolutely discover it as part of the wrap-up to an encounter.
Example: PCs fight-off and destroy a bunch of robots. At the end, when describing the room - there are usually two methods: one, describe the things the players obviously see, and two, await search-related skill rolls.
- If it's a required clue (such as, the entire region will be vaporized, thus the end of the PCs), then at the end of the fight, you will describe: "Among the refuse of robots, you can plainly see a number of components, now visible thanks to the frayed chassis being blown open.... wiring, blhablahblah bomb bomb bomb"
- If it's not a required clue (the resulting explosion may be lethal, but, more likely to simply be a setback for the PCs) - then, it can be found as a result of a search check.