How can Shi'a Muslims know which Imam to follow?

All factions of Shi'a Muslims, despite their myriad differences, agree on the fundamental concept of *Imamah*: That the authority of leading the Islamic state is divinely-granted, and that not following the rightful Imam is disobedience to God's authority. I was reading this article — clearly anti-*Imamah* and likely biased — and it brings up some very good points, namely that while every Shi'a sect may have its own evidences to prove that its Imam is the "correct" one, these evidences are not universally accepted, and even outright rejected by other Shi'a factions (obviously, otherwise they'd all be following the same Imam). The Qur'an is mostly silent on the exact succession of leadership or how it is to be decided, and this is an issue that's been hotly debated since the death of the prophet himself. Given that one accepts the Shi'a concept of *Imamah*, how can one be sure that he is following the correct Imam given the fact that there exists about 70 different branches of Imams or representatives(detailed here ), each branch split over the same succession issue as faced during the time of Prophet Muhammad pbuh's death? What criteria are available for determining who is to be the actual Imam of our time?