That's actually a very good question. What would it take to convince an atheist that something is God as opposed to some other explanation? I don't think it's possible to get that answer. There is always an alternate explanation in simulation theory. The holy spirit could just be a drug trip. There is always some alternate explanation, and so it's futile to even attempt to know whether or not God exists. That doesn't mean we should stop looking. Maybe we can know whether or not it's the alternate explanation.