We know the lost tribes are lost. We say, they are lost, but they are not in some hidden valley or someplace under the earth’s crust or on some distant star. The lost tribes are "scattered" among every nation, kindred, tongue, and people. In what senses are they lost? They are lost to the lands of their inheritance. They are lost to the gospel and the saving ordinances. They are lost to the priesthood and the blessings that flow from it. They are lost in the sense that many do not even know that they are of the house of Israel. One of the reasons we send our missionaries to the four corners of the earth is to find, gather and expand the house of Israel.
Then how can they know that they are Israel? They come into the church. Passing through "the waters of baptism" (1 Nephi 20:1), they can next receive the gift of the Holy Ghost. They can receive a patriarchal blessing from an ordained patriarch who declares to them that they are of one of the lineages of House of Israel, and these days patriarchs have pronounced lineages of all of the twelve tribes. Historically, the majority of the members of the Church have been declared to be of the tribes of Ephraim or Manasseh, but now there are members of the Church from all of the different tribes of Israel.
In verse 5, Nephi states that Israel will be scattered among all nations. He knew that the northern ten tribes were conquered by the Assyrians in 721 BC and taken into the north and then scattered. The southern kingdom of Judah was taken captive by Nebuchadnezzar and the Babylonians in 587 BC. Then some were allowed to return so at the time of Christ there were Jews in Judah, but Jerusalem was destroyed and the Jews were scattered by the Romans in AD 70, and Israel was further scattered all over the world. Nephi saw this as the result of their hardening their hearts "against the Holy One of Israel" (22:5). We would say that Israel was scattered, dispersed and disorganized because of apostasy. Nephi also states that ultimately the seed of Lehi will likewise be scattered (22:7) and "dwindle in unbelief" (as Nephi had seen in 1 Nephi 12:20-22).