How did Sai Baba prove to Cholkar that he was aware of his secret sacrifice involving sugar?

After Cholkar finally arrived in Shirdi and fulfilled his vow, he was staying as a guest with Mr. Jog. As Cholkar was about to leave Jog's place, Sai Baba, who notably did not drink tea himself, gave a very specific instruction. According to Chapter 15, Baba told Jog to give Cholkar "cups of tea, well-filled with sugar." Hearing these meaningful words, Cholkar was amazed and moved to tears, as it was a direct and undeniable sign that Baba was omniscient and knew all about his secret, long-held vow to abstain from sugar.