How did the incident with the grapes demonstrate Sai Baba's omniscience to Boss Dharamsee?

The incident demonstrated Baba's omniscience by showing He was aware of Dharamsee's most private thoughts and dislikes. As Chapter 35 details, Dharamsee silently questioned Baba's sainthood in his mind, thinking that a true saint would know he disliked seeded grapes. Immediately following this thought, Baba gave him even more grapes, directly challenging his doubt. The ultimate proof came when Dharamsee, upon eating the grapes, found them to be seedless. He then realized the truth, thinking, "Knowing the intent of my mind, although they were with seeds and unwashed, Whatever Sai gives me, they turn out to be seedless and beneficial." This proved to him that Baba knew his inner state completely.