“How do we know it is true?”
God anticipated that in the future, there would be skeptics who wondered if the Exodus story was true. God dealt with the unbelief with two monuments: one at the Jordan’s edge, made of twelve large stones that had been taken from the Jordan river bed, and another at Gilgal, made with twelve similar stones. These were huge rocks, big enough that it would need to be hefted onto a man’s shoulder to enable him to carry them.
From this point on, when people saw the monuments (which remain to the day of the book’s author), they always remembered what they represented: God’s miraculous action against the Jordan river – an action similar to His action against the Red Sea vs. 23).
The phrase “to this day” does not mean “to our present day,” meaning yours and mine, but only to the day of the author of the book. It occurs fifteen times in Joshua, and 83 times in the Old Testament as a whole (though Joshua uses it most of all). It does not offer insight as to when the “day” was the author is talking about, but it does tell us that some time had passed since the events and the author knows the stories of these monuments, as well as the fact that they have been there a while. God does not leave Himself without witness.