There are a lot of things that, if they happened, would convince me that god exists without the shadow of a doubt.
If everyone all over the world had the same dream, on the same night, of the same god, explaining his existence, and what he wanted. And if all the visions were the same (Obviously I wouldn't need the confirmation of everyone on the planet though.) I would be convinced.
If he gave me a vision of something major that was going to happen, along with an exact date. Something like 9/11 and the twin towers, or hurricane Katrina. If it wasn't vauge, I would be convinced. (You know a simple, "LtCmd.Lore, in exactly 2 months and 6 days a major earthquake will strike New York City." Or even something smaller: "LtCmd.Lore, in exactly 3 months and 2 days your uncle will give you a 50 dollar gift certificate to Radio Shack. Tell no one!")
Of even if he just appeared to me, while I was perfectly sober, and while I was surrounded by a large amount of perfectly sober and unbiased witnesses. Or if he appeared to me, or spoke to me regularly like he did with Moses.
In fact, for an all powerfull diety it wouldn't be that difficult to make me believe in him...
But whether he exists or not it would take quite a bit to convince me that he's worthy of my respect, let alone worship... After thousands of years of letting humans suffer and die, a simple "Hey, I do exist by the way." is hardly enough to wipe the blood off his hands. He'd either have to come to earth begging for forgiveness, or have a pretty darned good excuse.
http://www.ebonmusings.org/atheism/theistguide.html
LtCmd.Lore