On the basis of what evidence did W.M.L. de Wette conclude that Deuteronomy was composed in the seventh century? One answer is that de Wette wasn't basing his conclusion on evidence and rational thought; rather he was driven by Romantic notions of authorship and anti-Jewish tendencies to arrive at his invalid conclusions. (See posts and comments here and here for background.) That would be the wrong answer according to Israeli scholar Alexander Rofe.
[T]he law of the unification of the cult, which appears over and over again in D between 11.31 and 31.13, became a tool for dating D and, by extension, for dating all pentateuchal literature. The dating, following the lines proposed by De Wette (1805), is based on the following historical syllogism: (1) of all the books of the Pentateuch, only D commands the unification of the cult; (2) the unification of the cult was carried out only twice, the first time not on the basis of a written law, in the days of Hezekiah, King of Judah (727-698), and the second time on the basis of a book of Torah that was discovered in the temple, in the eighteenth year of Josiah, King of Judah, that is, 622; (3) it follows that the book that was discovered was the D document (or part of that document), and that it was written in the seventh century, between the cult-unification activities of Hezekiah and those of Josiah. Henceforth pentateuchal literature can be dated based on its relationship to D. Documents that are unaware of the unification of the cult must predate the seventh century, and documents that assume the unification of the cult must post-date it, from the time of the exile or the return[.] (p. 4)Reference: Alexander Rofe, Deuteronomy: Issues and Interpretation, London: T&T Clark Ltd., 2002.