It's good enough that it has been done a lot of times. Just make your story good -- better than the others. It doesn't have to be an original plot to be a blue-ribbon winner for you.
It depends on how intelligent I guess they might be. There noting to gain by arguing with a fool. And if the person is reasonably intelligent, it might be fun to have a discussion. But I really don't give a flip what people think about my spiritual/non-spiritual identity. If I had to count on...
One clue is that a true personal prophecy will confirm what you already suspect. Anyone who tells you something you have never thought of is a phony. There are other tests, but that is one of the simplest.