In a traditional "mirror universe", depicted in fiction, good and evil are swapped, blah blah blah. But has anyone ever noticed that not everything is reversed? What is considered good and evil is just the same as the normal universe, just the people who act on it are considered the opposite of what they usually are; for instance, the Pope, who is considered good, in a mirror universe would considered evil and strive to do the evil thing, but in a traditional "mirror universe", an act like murder - which is considered evil in our universe - is still considered evil, so the now evil Pope would expound the glory of killing.

But wait! If everything is opposite in a mirror universe, then the sense of morality for what is good and evil - the very concept of it - would be reversed, as well, so what we would consider evil in our universe would logically be considered good in theirs: murder would be considered good in a mirror universe, instead of being evil, like it is in our universe. So, logically, taking our "evil Pope" example, he wouldn't be expounding the virtues of murder, he'd be arguing against it, because to him, murder is good instead of evil, and he doesn't want to be associated with anything he considers good. So, if that's the case, then functionally, a mirror universe would act no different from the normal universe.

And if that blows your mind, consider this: this concept of opposites extends to the NAME, as well; a mirror universe would name something the opposite of what they consider it to be, so they wouldn't call something that they consider evil evil, they'd call it good. So, logically, what is considered good here would be considered evil there, but would be called good there, per the law of opposites.

So, taking that all into account... if we ever do come across a "mirror universe", how would we tell the difference?