No one is sure, but we can make educated guesses. We know that the IRV society did not follow the Vedic traditions of horse & fire sacrifices, but instead we more focused on a fertility/goddess worship. We know from the excavated remains of Harrapa and other cities that their public buildings surpassed other civilizations at the time. We know they did extensive trade.
So from that, we can surmise that they had a religious hierarchy, a public works bureau (in some capacity), and a fairly decent grasp on "foreign policy." More likely the folks in charge of the growth of public buildings were the landed wealthy in the area, as is common even today.
With this in mind, we can guess it was likely a theocratic oligarchy, with the religious folks as figureheads, but with the wealthy class pointing out the directions the society would take.
