Well, we can make judgements about what's most likely. For example, if we see a crop circle in a field, we can consider what the most likely cause is. We may not be able to prove outright that there is a supernatural or extraterrestrial cause, but it's far more likely that humans did it. Of course, if we see evidence one way or the other, that proves the cause. But until such evidence is seen, we can go with the most likely cause.