How do the LDS confirm the first vision/Moroni without having anything to test it against?

I've been looking more and more into the Church of later day saints. I'm obviously not a member, but I am curious about them all the same. My question is about how it appears as if the majority of their answers regarding the first vision* or their angel Moroni are sourced from those entities themselves. Edit: *The first vision as far as I know was experienced only by Joseph Smith. https://christianity.stackexchange.com/questions/91367/how-do-christians-discern-between-genuine-dreams-visions-from-god-and-false-dr I saw this answer regarding how the LDS are supposed to determine if a vision, angel, etc is from good or evil. And I agree that if the original source was valid, it would all be okay. But from my understanding all the sources listed by the LDS answer are in and of themselves from the beliefs delivered by Moroni or aligned with them in new teachings. How does an LDS believer know these teachings are valid in the first place and not trick? (Or is this the faith part?) Edit: For clarity, I'm not asking for the current teachings of the LDS, I'm asking how is the basis of the LDS faith confirmed as "divine" before their teachings confirmed it? For example: An angel gives you a special book, the book says the angel is definitely not evil. (A proves B, B proves A) How do you prove the source is valid without using the source's own claim?