The narrator in the poem hears the report of the statue ruins from "a traveller from an antique land."
The implication in the poem is that this traveller is assumed to be one who understands the significance of the statue remains and the inscription found there. The traveller says that the sculptor of the statue did a good job of portraying the "sneer of cold command" that Ozymandias must have often displayed. Indeed, the disdain with which Ozymandias regarded those around him, indicated by his comment, "Look on my works, ye mighty, and despair!" is still conveyed through the form of the pieces of the statue that remain.
The narrator assumes that the traveller has seen and experienced enough of powerful rulers from other places to be a reliable authority in interpreting the meaning of the rubble in the desert.