I also can’t think of any way it could be done that would rule out fakes or delusion. If I saw a being doing things like shift stars to form words, I’d be more inclined to think I was going insane or that we are all in the Matrix than in a God. Both are much more plausible.
Although someone running a simulation with us in it might qualify as a “God” by the OP. Even if he’s just an alien computer programmer.
I think the point about the message in the stars is that everybody would be able to see them just by looking up. Which would probably convince you after a while that you’re not just going crazy.
As far as a simulator is concerned, from the point of view of the creator, would there be any difference between a god that creates a universe and everything in it, and a programmer creating a simulation that from the inside is indistinguishable from a “real universe”?
"WE APOLOGIZE FOR THE INCONVENIENCE, " in flames, on the side of a mountain might be nice.
“Stick your head in a pig” works for me.
I would think a good test for an all knowing, all powerful being would be: Can you think of anything that you could do, within your power, that might be noticable to the average person as supernatural?
If he needs advice from the SDMB on how to get noticed then he has already flunked the all knowing part. Perhaps, with enough good suggestions here, he will have the chance to show himself as all powerful.
I don’t have free will with regards to what I believe is true. Do you? Are there things that you do not believe in, then through an act of will, believe is true?
If we presume the existence of an omnipotent diety who can harden the hearts of Pharaohs and such (something which never made sense to me, by the way), then by definition we only have what free will He allows to have, i.e. effectively none because it can arbitrarily be taken away at whim.
So if the biblical God wants to prove Himself, let Him alter my mind to believe in Him, if He can. Otherwise, fancy tricks like rearranging stars speak to hyper-advanced technology, not divinity.
Let’s dispense with the all-knowing part. If I ask a putative god a thousand questions and he gets them all right, it doesn’t prove that he’s all-knowing. It only proves that he answered a thousand questions correctly — and that’s presuming I have the ability to verify his answers are correct. I can ask him a trillion questions about things on Earth, which I can verify, but that wouldn’t prove he’s all-knowing, which by definition includes about a trillion googol things not on Earth. What’s the temperature on Mugwug III? God would know. But how do I know that God’s right?
As for the all-powerful part, Q from Star Trek would surely be close. If Q showed up and said he made the Earth, would you believe he’s God?
No. The universe is not so badly designed.
Which you don’t; if he’s “all powerful”, he can certainly screw with your mind so you believe whatever he tells you. It’s another aspect of the problem mentioned earlier; how you can’t distinguish a “god who created the universe” from something that’s just really powerful but lying.
If I’m crazy enough to see stars turn into words, I’m crazy enough to hallucinate people agreeing with me, I’d think.
I’d expect the best way for God do prove he exists would simply be to show up. In a universe that had a God, it would sure be expected that every being there would know without question that He exists.
So my answer is to show up regularly, sort of like God did to Arthur in The Holy Grail.
What would his appearance have to do with free will? We’d still have it just as much as we did before his debut. I don’t understand the question.
I think I am going to become a polytheist because it is increasingly clear to me that the Universe was designed by a committee.
Rob
If nothing is beyond the power of said god, I’d want to know everything it knows. Understand everything it does. Since anything and everything is possible, make me a god. Then I’d buy it. Otherwise, I’d suspect we were all being conned by very advanced creature(s) of some sort. Technology may or may not play a part in their con job.
I can think of many things that would prove there existed something far more powerful than I previously thought existed.
I cant think of anything that would ‘prove’ god as such, given we can already hypothesise ways to directly impact our senses and make us think we’re seeing or remembering something that isnt real.
I guess with enough power you’d have God for all practical purposes, regardless of the truth. They could make me believe they’re god after all, and Id know no better.
Otara
Good idea, O Lord! 
The difficulty with such a question has been pointed out pretty well: How would we be able to distinguish between an advanced technology, or that we’re going crazy (etc.)… and a god (and either is more likely than a supernatural being)
I’m with Ponchacco above. Atheists* have pretty clearly established that there is no current evidence supporting a supernatural anything, the less clear part is why theists believe their god the “correct” one?
And, along those lines, why do they not believe any of the other gods exist? Understand this alone and you understand much about atheism.
In theory, all universes are possible. In one of the possible universes, everyone has free will and always makes the right choice. That’s the universe any worthwhile god would have created.
As my ex boss used to say, “I believe in the entire old Norse pantheon. Surely you don’t think all this wonderful complexity could be the result of just one god…” 
Is this being claiming to be both omniscient and omnipotent? If so, I would ask him (it?), once he’s decided on a course of action, whether he’s able to change his mind.
If “yes,” then he’s unable to foresee his own future actions, so he’s not omniscient.
If “no,” then mind-changing is something he can’t do, so he’s not omnipotent.
There can be no contradictions; therefore, nothing can be omniscient and omnipotent.
If an entity of some sort descends from the heavens, looks and/or points directly at Fred Phelps, says “BLASPHEMOR!” and then Phelps spontaneously combusts, along with all his followers…I’ll at least be williing to consider the possibility that said entity is God…or an otherwise Righteous Dude.
There are some very simple thins God could have done to make believing in him more reasonable:
Give his word to all his children, not just a small group of people in one part of the world. It should be easy enough to give his word to the ancients of Mexico, China, Hawaii etc. If copies of the Bible were found in on every continent before they were “discovered” I’d more readily buy that it is something special.
If God sent his son to die for our sins, couldn’t he have sent other versions to other parts of the world.
Why do other groups of ancient people have to wait hundreds or thousands of years to hear the good news? Apparently God Needs his children to worship him, so why not contact everyone rather than a very small population.
Surely when God inspires someone to write his words, he could have also inspired them to get it right and have it make sense. You can’t even get out of the first two chapters of Genesis without glaring contradictions.
When exactly were man and woman created? What order? When were the stars, animals and the like created?