There are actually quite a few rules given to determine if a messenger is from God. Jesus for instance said “If any man will do his will, he shall know of the doctrine, whether it be of God, or whether I speak of myself”, then there is the qualification in Deuteronomy, the requirement in John, the experiment in Alma, the promise in Moroni, and some details in the D&C. It is somewhat of a bootstrapping problem as one must already trust one of those sources, or the person presenting those sources, enough to move forward in trying to verify the source and messenger.
God obviously.
There are actually quite a few rules given to determine if a messenger is from God. Jesus for instance said “If any man will do his will, he shall know of the doctrine, whether it be of God, or whether I speak of myself”, then there is the qualification in Deuteronomy, the requirement in John, the experiment in Alma, the promise in Moroni, and some details in the D&C. It is somewhat of a bootstrapping problem as one must already trust one of those sources, or the person presenting those sources, enough to move forward in trying to verify the source and messenger.