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?
In the case of a disputed election, the decision is largely based on which pope is elected first, although this is not always the case. Eulalius was elected pope in 418, one day before Boniface I. However, through overconfidence, Eulalius flouted an imperial order and the emperor then recognised Boniface as the rightful pope, leaving Eulalius as antipope. So, in this case it was the emperor who made the decision.

Ursicinus (or Ursinus) and Pope Damasus I were elected simultaneously in 366. Ursicinus ruled in Rome for several months in 366–367 but was afterwards declared an antipope. As described by Adrian Hastings, in A World History of Christianity, page 42, Damasus succeeded by hiring a gang of thugs that stormed the Julian Basilica, carrying out a three day massacre of supporters of Ursinus in order to have himself elected pope. Damasus was since regarded as the true pope, reigning from 366 to 384.

Boniface Franco had murdered the reigning pope, Benedict VI, and was elected as Boniface VII in 974, so at this stage he was presumably a legitimate pope. However, when Emperor Otto II invaded Rome, Boniface fled to Constantinople, and Otto arranged the election of Benedict VII. When Otto died in 984, Boniface returned to Rome and imprisoned Pope John XIV, who had succeeded Benedict VII. Boniface was re-established as pope, but died in suspicious circumstances. Boniface VII was not yet considered an antipope when Benedict Gaetani took the style of Pope Boniface VIII in 1294, thus tacitly recognising the legality of Boniface VII. However, Boniface VII is now regarded by the Church as an antipope, and there is no Pope Boniface VII in the list of popes.

Change can also result in former antipopes becoming recognised as popes. Leo VIII, was for many years considered an antipope but is now listed on the Catholic Encyclopedia as a true pope. 

The Western Schism began following the election of Urban VI in 1378. As a candidate who was not yet a cardinal, it was thought that he would be a compliant pope who would allow the cardinals the freedom to which they had become accustomed, but sudden power turned him into a violent egomaniac.  The cardinals soon called on him to abdicate and elected Clement as his replacement.  The following year, Clement fled to Avignon with a group of loyal cardinals, and the two popes excommunicated each other. Finally, there were three claimants to the papacy, John XXIII, Gregory XII and Benedict XIII. A council set up to resolve the issue secured the resignations of John and Gregory, and excommunicated Benedict when he refused to step down. John W. O’Malley, S.J. says, in A History of the Popes, page 150, that the Catholic church has never made a formal pronouncement on the claimants, but especially in the past hundred years the Holy See has in the lists of popes it regularly publishes always sided with the Roman over the Avignonese line.