3 Nephi 5:2

and they knew that it must be expedient that Christ had come, because of the many signs which had been given, according to the words of the prophets; and because of the things which had come to pass already, they knew it must needs be that all things should come to pass according to that which had been spoken;

They knew Christ had come because of the many signs that had been given. These signs matched what the prophets had said. The things that had already happened proved that everything the prophets said would come true.

All the signs they’d witnessed proved that Christ had come, just as the prophets said he would. Everything that had already happened showed them that everything else the prophets spoke about would happen too.