In the past there have been times when there was a real pope and another person claiming to be the pope, who we call the anti-pope. In some cases the real pope didn’t live in Rome. So how could we tell which one is real, for sure?
Today it’s the same problem, Pope Benedict XVI lives in Rome, and Pope Pius XIII lives somewhere else, but claims to be the legitimate pope. How do we tell for sure which one is the anti-pope?
I know it seems obvious because one has so much more following than the other. But in cases in the past the anti-popes have had lots of support.