Actually, prophecy is pretty easy in hindsight. The oracle at Delphi, according to its own PR, had a much more impressive track record then either Judaism or Christianity.
Edit: You show your ignorance. Not only did the Oracle at Delphi have a millennium and a half track record of making prophecies, but the Sybil of Cumae supposedly accurately predicted the future of Rome from Kingdom to Republic to Empire. It was a prophecy that told King Byzantius where to found his city, a city that would become one of the most important Christian cities for a thousand years. And common English words for prophecy, like augury, comes from Etruscan mysticism.
However, your critique of vagueness regarding pagan prophecy is justified, but in that it is just like biblical prophecy, the vague wording means it can be back engineered into history whenever and however you want.