How could it really be proved wrong?
I would say it can be proved wrong by the fact that, according to canon law, the Sacrament of Ordination requires proper Form and Intention among other things. If these requisites are not met there exist canonical "impediments" which render the sacrament null and void. Since the time of St. Peter ordinations to be valid required a MALE as a candidate for ordination. Thus, if in fact a woman was "ordained" this would be be an invalid and utterly meaningless act. Since there can be no female priests there can't be a "Pope Joan". I won't go into all the theology of ordination to holy orders but I have some first-hand personal (and very modern!) experience of this sort of blasphemous action. (and blasphemy according to word etymology means "bull" not sacrilege) Would anyone like to hear about it?
Until then, you might want to peruse this list of popes from the Prophecies of St. Malachy
Some people are staunch believers in this list. At this time I lean that way.