I personally doubt the existence of a God, but I'll be the first to concede that there's no way to know for certain either way. I believe in evolution, but even that doesn't automatically rule out the existence of a higher being. Though I would call myself an atheist, the science-based "primordial soup" theory of what first sparked replicating life is a little vague even for my taste. It's entirely possible that there is a wholly scientific explanation for what began life as we know it, that we have yet to stumble on or comprehend but may someday discover as technology and our understanding of the natural world progresses. However, the idea of a higher being having given life to this first spark, and then leaving things alone to develop as chance would have it, is also entirely feasible and would not be out of line with the rest of my beliefs. I believe in science, but science right now can't explain to me how basic chemical substances that originally existed in fixed amounts found ways to multiply themselves. However, the idea of a higher being providing that first spark and then simply sitting back to watch the domino effect doesn't mesh very well with any well-known, well-established religion that I know of, and certainly not Christianity, which is the religion followed by the rest of my family.
Back to the original topic...there is no scientific experiment I or anyone else can conduct (or at least none that anyone has yet conceived of) to "prove" or "disprove" the existence of a higher being. I agree with Silent Song--whether one believes in God or not, the decision is faith-based either way.
