At the end of Helaman 8, Nephi had prophesied that they would find that the Chief Judge had been murdered, and that his brother had killed him. That was a pretty specific and testable statement. It is unsurprising that many hurried to discover whether or not it was true. As it says in verse 2, “now we will know of a surety whether this man be a prophet of God.”
They do find the Chief Judge lying in his own blood, and therefore felt that they had received confirmation that Nephi was a prophet, and, if he had been a prophet in this, he would be a prophet in the calamities he had prophesied, except they should repent.