Alma 49 follows what the Lamanites were doing. But how would the Nephites, who wrote this all down, know what the Lamanites were doing? Did the Nephites invent remote mind-reading? That seems pretty unlikely, but I have a few, more probable, ideas.
Maybe the Lamanites kept their own record which eventually fell into Nephite hands. But the Brief Explanation says the only plates used were
The Small Plates of Nephi …
The Plates of Mormon …
The Plates of Ether …[and] The Plates of Brass.Brief Explanation about the Book of Mormon
Maybe the Nephites were just guessing what the Lamanites thought. But how would they know about this?
And it came to pass, that when the Lamanites saw that their chief captains were all slain they fled into the wilderness. And it came to pass that they returned to the land of Nephi, to inform their king, Amalickiah, who was a Nephite by birth, concerning their great loss.
And it came to pass that he was exceedingly angry with his people, because he had not obtained his desire over the Nephites; he had not subjected them to the yoke of bondage.
Yea, he was exceedingly wroth, and he did curse God, and also Moroni, swearing with an oath that he would drink his blood; and this because Moroni had kept the commandments of God in preparing for the safety of his people.Alma 49:25-27
Maybe they were spying on the Lamanites and heard them talking about this. But, in my opinion, the most probable way is asking converts or prisoners in verses such as
And it came to pass that the army which was sent by Moroni, which was led by a man whose name was Teancum, did meet the people of Morianton; and so stubborn were the people of Morianton, (being inspired by his wickedness and his flattering words) that a battle commenced between them, in the which Teancum did slay Morianton and defeat his army, and took them prisoners, and returned to the camp of Moroni. And thus ended the twenty and fourth year of the reign of the judges over the people of Nephi.Alma 50:35
Now behold, this was the desire of Moroni. He took them prisoners of war, and took possession of the city, and caused that all the prisoners should be liberated, who were Nephites; and they did join the army of Moroni, and were a great strength to his army.Alma 55:24