In Catholic understanding when there is more than one Pope, who decides which Pope is real and by what criteria?

I know at various periods of history cardinals were split submitting to multiple competing popes. However as they die, eventually only one Pope is considered the 'real' infallible Pope, and the others labelled as 'anti Popes'. My question is aside from the view of each Popes view, that the others are not 'real', who finally determines the 'real' one and by what authority? I mean does a certain group decide? Do you just wait until there is only one Pope and then he determines how to label the other Popes because now only his word stands? Who decides which Pope is real and by what criteria? Does the moral behavior of the candidates have any relevance in the criteria?