After Nephi retrieved the plates of Laban, he notes that his father Lehi took them and searched them from the beginning. Among other things, Lehi "also found upon the plates of brass a genealogy of his fathers; wherefore he knew that he was a descendant of Joseph: yea, even that Joseph who was the son of Jacob, who was sold into Egypt" (1 Nephi 5:14). Critics of the Book of Mormon have wondered how a devout Jew could fail to know what tribe he was from until he saw the plates.
Charles Pyle replies with the following question, "How many devout Jews today know, by heart, their genealogy, or what tribe that they are from? Sometimes, historical circumstances make it such that a genealogy may be forgotten, or the details of their tribal lineage may not be remembered correctly. Such was the case with certain individuals who were of the priestly line, who required a tracing of the genealogies, only to find that they had been omitted from the genealogy of the priesthood (Nehemiah 7:63-64). Nephi's comments could easily lend themselves to the interpretation that Lehi, upon actually seeing his lineage, then knew, beyond oral tradition, of his line of descent. Before Nephi even was sent to retrieve the plates, Lehi proclaimed that "Laban hath the record of the Jews and also a genealogy of thy forefathers, and they are engraven upon plates of brass" (1 Nephi 3:3; see also 3:12). [D. Charles Pyle, "Review of 'The Book of Mormon Vs. the Bible (or Common Sense)'" [http://www.linkline.com/personal/dcpyle/reading/bodineco.htm], p. 4]
Note* Perhaps Lehi found that he was a direct descendant of Joseph. In other words, that Nephi was entitled to be the caretaker of the plates because of his blood lineage. [Alan C. Miner, Personal Notes]