Therefore the people of the Nephites were aware of the intent of the Amlicites, and therefore they did prepare to meet them; yea, they did arm themselves with swords, and with cimeters, and with bows, and with arrows, and with stones, and with slings, and with all manner of weapons of war, of every kind.
How were they aware?
The scriptures don’t tell us, but I have some ideas:
1: The Nephites had a spy that told them.
2: The Amlicites mistakenly did all that in plain sight.
3: The Amlicites accidentally told the Nephites.
4: The spirit told them.