No Marvel character has been present for as many truly momentous events as Uatu, aka the Watcher. Just one member of a race of powerful, ancient beings, Uatu is charged with observing the events of Earth's solar system, giving him a front-row seat to all of Marvel's most important stories. The Watcher is such a significant presence that his appearance is often the first clue that things are about to get serious. But with that being the case, it's fair to ask how Uatu knows what's going to be important enough that he has to turn up in person.
Created by Stan Lee and Jack Kirby in 1963's The Fantastic Four #13, Uatu's title as the Watcher is a little misleading. In fact, Uatu is just a Watcher; one of a far larger race who, after meddling with sentient species at the beginning of the cosmos and seeing their efforts end in disaster, swore to only ever stand as mute record-keepers of history. Of course, such promises are made to be broken, and Uatu has meddled with events on Earth many times; helping the Fantastic Four fight off Galactus, intimidating the Stranger, and even chastising the X-Men's Beast over his reality-warping experiments.
One of the biggest ways Uatu meddles with events on Earth is simply through his presence. Many Marvel characters now know the Watchers' function, so when he appears at weddings, deaths, or even just meetings between heroes (as he did during Marvel's Civil War event), people tend to take it as a cue that whatever's happening will have big consequences down the line, and they change their behavior accordingly. Of course, in making this assumption, they're also assuming that Uatu somehow knows what's going to happen. In fact, it's likely that he does, but not for the reason many fans assume.
The most important thing to know is that Watchers as a race cannot see the future. What they can do - similar to Asgardian God Heimdall - is see pretty much everything that's happening in their assigned area of focus. Uatu can directly experience almost every event in Earth's solar system (barring those protected by incredibly high-level technology or magic), and he's been able to do so for most of history. In this way, Watchers as a species tend to operate entirely according to guesswork, but it's guesswork informed by millennia of experience. Uatu confirms as much in Black Panther Annual #1 by Reginald Hudlin, Ken Lashley, and Larry Stroman, telling one of King Solomon's Frogs that he knew Black Panther and Storm's wedding was a momentous event simply because he understood its context in the wider world.
It's a neat explanation for the Watcher's past appearances, and even adds context to his role as the frequent narrator of Marvel's What If...? stories, which involve the Watcher relating what might have happened if the events of famous Marvel moments had occurred slightly differently. Despite that, it doesn't explain everything - for instance, the crossover series Beyond showed Uatu waiting by the grave of fallen hero Gravity, confident his story wasn't over. Educated guesswork is one thing, but what made Uatu so sure that events would conspire to bring a specific hero back from the dead?
The answer comes in 2020's Symbiote Spider-Man #1, in which Uatu is captured by Kang the Conqueror, who intends to use his futuristic technology to steal the Watcher's vast cache of memories. Uatu quickly turns the tables, using his mental powers to plunder the time-traveling despot's own store of information. Because of this event, it turns out that while the Watchers in general have no ability to see through time, Uatu's own adventures have given him access to some limited future knowledge, helping to explain those moments where his intuition couldn't quite be explained by guesswork alone.