How did Dev eventually find out that the Sanyasi who attended his ceremony was in fact Sai Baba?

Dev discovered the truth through correspondence with Jog. As detailed in Chapter 40, after feeling slighted, Dev wrote a letter to Jog expressing his disappointment that Baba seemingly did not attend the Udyapan. Jog communicated the contents of this letter to Sai Baba. In response, Baba explained that he had indeed been present in the guise of a Sanyasi with two companions. He firmly stated, "for the sake of My word, I would die; but the words from My mouth shall never be otherwise." Overjoyed, Jog wrote a detailed letter back to Dev, relaying Baba's account. Upon reading this letter, Dev's "eyes overflowed with a surge of love," and he felt immense shame for having wrongly blamed Sai.