If you're going to claim that you can see the future, the first task you face is being able to actually predict the future. That seems reasonably basic; you can't claim to be a spinal surgeon without proof that you know what's going on in the human back, for example.
Odds are good that you don't actually have the power to see into the future, since human beings have proven relentlessly incapable of that. So your goal for establishing in-character credibility is going to have to take a few twists and turns, starting with a time-honored modern tradition of looking at something that already happened and declaring that it lines up with a prophecy you already made.
If you're starting a new character who can predict the future but has never actually done so, your character is going to seem more unbelievable. That might be your goal in the first place, but if you're aiming for "viewer of the future" rather than "charlatan," it might not be the best goal to have.
Predict the certain
Some things aren't really prophecies, but they sound as if they are. Your character could, for example, predict that in the future, the war between the Horde and the Alliance will grow more vicious once again in World of Warcraft. The odds of this happening are pretty much absolute because you've got to get those PvP maps into the game somehow, but it has the advantage of being a vague pronouncement about the future that can be neither proven nor disproven until the next expansion comes out.
These sorts of prophecies have the advantage of helping to establish your character's credibility without doing any real heavy lifting. You know you're right, but you aren't relying on another player to make you right. Yes, depending on the game, this is sometimes obvious to people who don't have the gift of seeing the future, but predicting the obvious seems like a pretty basic test for anyone claiming to see the future anyway.
Aim wide, aim uncertain
Having your character predict what you as a player know will happen is pretty easy. Having that same character predict what may or may not happen is more dicey because things might go wrong. Very wrong. You could weave an intricate prophecy about someone only to find that the player winds up quitting the game in a week, invalidating all of the very specific allusions you made in the first place.
The trick, then, is not to do that. Instead, you just make things up.
We humans love to find patterns in things. That means that sufficiently vague prophecies can be lined up with almost anything, given time and enough motivation to shave off the jagged edges. Don't predict that Kyle's character will die in a duel with his rival; predict that a great wolf will be wounded by another of equal stature. It could refer to Kyle's character if that event takes place, or it could refer to a clash of nations or even a pair of actual wolves going at it outside the city gates.
When you can't be certain your prophecies will be right, you can do journeyman service to ensuring that none of them is wrong. Granted, that might be because most of them are vague to the point of uselessness, but your obligation with a prophetic character is to foretell the future rather than give anyone the tools to forestall or correct it.
Write ahead of time
So you've done all of the above and your character is accepted as a genuine source of prophetic knowledge. Every so often, you can use this with your character (and other characters, with their permission) as a source of some pre-planning that's indistinguishable from normal play.
For example, your character might mention, casually, that she's going to be injured in a week by a knife. A week later, you conspire to ensure that some sort of knife-related injury crops up. Maybe she gets stabbed, or maybe she nicks her thumb while slicing cabbage, but the point is that she saw something and it came true with remarkable clarity and speed. You can plan what happens next and lay your cards right on the table without it ever being obvious that you were just making a few plans ahead of time.
And yes, if someone else lets you, you can do the same thing, although you probably shouldn't declare that Kyle's character will die in a month if you're not sure what Kyle has in mind. That's not nice to Kyle.
Feedback is welcome in the comments below or via mail to firstname.lastname@example.org. You knew I was going to say that -- I know. Next week I'm going to talk about siblings, and the week after that I want to discuss how to strike up a conversation.
Every Friday, Eliot Lefebvre fills a column up with excellent advice on investing money, writing award-winning novels, and being elected to public office. Then he removes all of that, and you're left with Storyboard, which focuses on roleplaying in MMOs. It won't help you get elected, but it will help you pretend you did. If you need a refresher, check out the Storyboard Library.