Question: How do we know from Isaiah when the Davidic Servant described therein is referring to Jesus Christ, or Joseph Smith, or some future person; or maybe all three at once?
Answer: First, having imbedded many literary features in his book that prove his prophecies form an end-time scenario—one that uses the history of his own day as an allegory of the end-time—Isaiah isn’t speaking about Joseph Smith’s day but about the time immediately before Jehovah/Jesus’ coming to reign on the earth. Second, Jesus is never spoken of in the scriptures as a “servant” because he is Israel’s God himself. Nevertheless, as Paul says, Jesus took upon himself the “form of a servant” (Philippians 2:7). That is because when atoning for his people’s transgressions he acted as a spiritual proxy savior of his people under the terms of the Davidic Covenant in the pattern of Davidic kings. These kings fulfilled the role of temporal proxy saviors of their peoples in which each functioned as a “servant” of God when obtaining their peoples’ physical protection or temporal salvation.
Third, in instances where Jehovah is speaking of his individual “servant” in the book of Isaiah—as in Isaiah 42:1–7; 49:3–10; 50:10–11; 52:13–15; 53:11–12; 55:3–5—or where Jesus speaks as Jehovah of his “servant” in 3 Nephi 21:10 (quoting Isaiah 52:10 and 57:18), he is speaking of his end-time servant. That is in contrast to Isaiah 53:1–10, where an end-time spokesperson for God’s people such as his servant is speaking about Jehovah/Jesus. That is also distinct from Jehovah’s speaking of his collective “servant” in the Book of Isaiah who are God’s people as a whole. In the prophecies of Isaiah, Jehovah calls and appoints his individual servant to prepare the way before him by restoring the house of Israel—the Jews, Ten Tribes, and Lamanites of today—to his covenant, gathering them from exile and dispersion and establishing Zion among them so that Jehovah/Jesus may come to reign.