Here's how I'd probably do it:
If the opponent isn't an enemy, I'd go with a Diplomacy check, opposed by a DC I would determined based on the target's degree of disbelief. i.e. if I'm trying to convince the guy that his house is on fire, the DC will be low, because he'll be afraid you're telling the truth. If, on the other hand, you're trying to convince him there will be an eclipse tonight, and he's never seen nor heard of such a thing, the DC might be 30.
If the opponent is an enemy, I'd go with Bluff/Sense Motive. The fact that you're actually telling the truth is irrelevent. You're trying to convince an enemy of something he doesn't believe.