Before we can believe him, a Navi needs to bring evidence that he really received prophecy. Usually this is done by making numerous accurate predictions. However, we can also believe the testimony of a prophet that someone else is also a prophet. The Rambam’s example is Moshe testifying to Yehoshua’s prophecy.
What is the basis of this testimony? Does the first prophet receive a prophecy that the second one is a prophet? Does the fact that Yehoshua was Moshe’s student matter? Moshe is compared to the sun and Yehoshua to the moon.
To speculate, perhaps the Navi is able to testify because he knows his student, and the student’s prophecy is an extension of his own. (Note that we call prophets in training “bnei neviim”). Moshe is compared to the sun and Yehoshua to the moon, indicating that his prophecy was a reflection of Moshes. Similarly, Elisha received the “Ruach” of Eliyahu. Perhaps, by knowing themselves they also know their student. Is a teacher the only one who can testify to their student’s prophecy? How well can a teacher know their students?