How did Dharamsee know for sure that the grapes were supposed to have seeds? Was he the only one who got them?

Dharamsee was able to confirm the miracle because other people also received the same grapes. After his profound experience of the grapes turning seedless in his mouth, he saw that Baba Tarkhad, another devotee sitting nearby, was also eating the grapes distributed by Sai. As Chapter 35 narrates, Dharamsee then asked Tarkhad, "How were your grapes?" When Tarkhad replied that his grapes did have seeds, Dharamsee was "extremely surprised in his heart." This direct comparison confirmed that the miracle was a personal one, intended specifically for him, and not a property of the grapes themselves.