He doesn't have to "know" it. As the Vicar of Christ on earth, the visible head of Holy Mother Church, with the authority to bind and loose, he alone has the authority to grant plenary indulgences and set the conditions there of. Which of course is just a drawing on the spiritual treasure of the Church made up of the superabundant merits of Our Lord, Our Holy Mother and the saints. The merits of the Passion and death of Our Lord are infinite, and these he left to the Church.
But of course, as a well catechized Catholic, I'm sure you knew this.
pascendi, the jurisdiction of the Church (binding and loosing) does not extend to the souls in purgatory: And whatsoever thou shalt bind upon earth, it shall be bound also in heaven: and whatsoever thou shalt loose on earth, it shall be loosed also in heaven. Catholic Encyclopedia ("Indulgences"):
To say that an indulgence of so many days or years is granted means that it cancels an amount of purgatorial punishment equivalent to that which would have been remitted, in the sight of God, by the performance of so many days or years of the ancient canonical penance. Here, evidently, the reckoning makes no claim to absolute exactness; it has only a relative value.God alone knows what penalty remains to be paid and what its precise amount is in severity and duration. Finally, some indulgences are granted in behalf of the living only, while others may be applied in behalf of the souls departed. It should be noted, however, that the application has not the same significance in both cases. The Church in granting an indulgence to the living exercises her jurisdiction; over the dead she has no jurisdiction and therefore makes the indulgence available for them by way of suffrage (per modum suffragii), i.e. she petitions God to accept these works of satisfaction and in consideration thereof to mitigate or shorten the sufferings of the souls in Purgatory.