It's possible something could lead to a discrepancy that you could measure if you knew exactly where to look. (I'd still want to see a non-vague theory for how

It's possible something could lead to a discrepancy that you could measure if you knew exactly where to look. (I'd still want to see a non-vague theory for how that could happen, but I'll put that aside.) But even if you knew where to look, you'd just get a value, and that value wouldn't tell you which side of the discrepancy you're on. If the universe had very subtle shifts in how things align, or did very subtle rounding of certain numbers, anyone stuck inside the universe without external knowledge would not be able to tell whether physics matches the pure mathematical expectations or not. I understand that there might be a threshold in some cases. But given how much of the universe is already effectively random when you get down into the itty bitty details, and that you can only make an experiment so big to tame the statistics, there could easily be untold rough edges that are invisible to us. Some particle went a different way than it "would have", but you can't measure "would have", it still looks like a normal interaction with a normal distribution of outcomes. Even if you had access to a parallel universe and you knew either you or them had 'correct' physics and the other didn't, I don't see how you'd be able to figure out which is which.