How did Sai Baba demonstrate his knowledge of Cholkar's secret penance?

Sai Baba's demonstration of his omniscience is the climax of Cholkar's story in Chapter 15. After Cholkar had fulfilled his vow by distributing sugar, he was a guest at Jog's place. As they were about to leave, Baba specifically told Jog to "Give him cups of tea, well-filled with sugar." This instruction was deeply significant because Baba himself did not drink tea, and more importantly, only Cholkar knew about his secret vow to abstain from sugar. This act left Cholkar amazed, with tears of joy, as it was undeniable proof that Baba knew everything—the original vow, the financial hardship causing the delay, and the secret, personal sacrifice of renouncing sugar. Baba later confirmed this, stating, "Your vow of sugar has been accepted, And your rule of renunciation is also complete."