When Antonio was born, it was Ultima that delivered him and buried the afterbirth. This is supposed to give Ultima a special insight - in addition to her other means of foresight - into Antonio's destiny because, knowing where the afterbirth is buried and in what specific conditions, directions, etc., she can foresee some of his future.
In addition to her insights, Ultima is also the most influential person in Antonio's life, shaping his character, his knowledge, and his sensibilities.
She becomes a mentor to Tony, helping him to interpret his dreams and negotiate the dangerous terrain of his quest for self knowledge.
Though she refrains from telling Antonio how and what to be, she is a very significant influence on him. In this way she both knows his destiny and helps to build it.
According to Professor Cantú of Cal State L.A., Ultima knows his destiny up to the point where she dies.
Professor Cantú makes the case that Antonio unwittingly serves as the person who betrays her, and that she knows all the while as she is teaching and mentoring Antonio that he will not act fast enough to save her when Tenorio Trementina goes after her the second time.
It is generally agreed among critics that Ultima becomes the heroine of the novel at that part where she dies to save Antonio's life.
Readers in the know (that is professional critics) agree that after Ultima's death Tony goes on and becomes a writer. They know this because the author, Anaya, has given many many interviews and made audio tapes in which he reveals the autobiographical nature of the novel. Anaya says that Ultima came to him in a lucid dream and told him that if he wanted the Novel to come together then he had to put her in it. He then states that Ultima was his guide and mentor as she was Tony's as well. Thus we can infer that the Author new Tony's destiny as well.