Can one understand a saint's true nature by observing their external behavior, like what they eat? Use the story of the Yogi to explain.

No, one cannot understand the true state of a saint by observing external factors like their caste, behavior, or what they eat. Chapter 23 explicitly states that their true state is always beyond such worldly measures. This is illustrated by the story of a practitioner of Yoga who came to Sai Baba seeking help to achieve Samadhi. When he saw Baba eating bread with onions, he developed a strong misgiving, thinking, "How can this man remove my doubts?" Baba, being omniscient, immediately addressed the Yogi's unspoken doubt by remarking that only those with strong digestion should eat onions. This startled the Yogi, who then surrendered with true devotion and had his doubts resolved.