How can the SLC branch of LDS PROVE that THEIR line is 'authentic'?
Leave the proving to God. Everything else, is just guessing, or worse seeking signs. But really, Elsie, we've been given ways to tell. Start with the fruits. Which tree has good fruit and which tree has none. Christ clearly taught this as a way to discern his followers. But He alone can give truth. Take the matter to him if you are truly interested in the answer. However, I predict you are only looking for further ammunition to do harm to others, and getting the quiet witness of the Lord with such motives can 'prove' difficult.