The simple action described in this verse belies the import of the event for all who must have faith. Ammon asks the queen if she believes, and she does. What she also indicates is that she has no reason to believe except the word of Ammon and the servants. In other words, there is no overpowering witness of the spirit present. There is no convincing argument that has convinced her. She simply has the word of Ammon and the servants, and their word is sufficient.
In this simple ability to believe based upon the word of another, the queen is laying claim to one of the most powerful of the gifts of the spirit:
D&C 46:13-14
13 To some it is given by the Holy Ghost to know that Jesus Christ is the Son of God, and that he was crucified for the sins of the world.
14 To others it is given to believe on their words, that they also might have eternal life if they continue faithful.
For some, the spirit comes to them directly, and it is their gift to have a personal witness of the truth. Certainly Ammon’s experience with Alma either qualifies or comes close. For King Lamoni, he was in the process of having this very direct and personal witness of the Spirit. While the verse in the Doctrine and Covenants is most specific about those who know that Jesus is the Son of God, the principle may be easily extrapolated to those who may gain an essential knowledge of any gospel principle through direct spiritual confirmation. Those people are indeed blessed.
There are others, however, whose ability to believe is much greater. They apparently do not need the personal confirmation of the spirit, but like this Lamanite queen, may sincerely believe because of her faith in the words of others. Of course this requires that one have faith in the others so that their words might be believed, but it remains a powerful and effective faith.