Not to mention that the language itself is wholly inconsistent with such an early datation. We know a great deal about the history of West Semitic languages thanks to inscriptions and other epigraphic finds. If "Moses" wrote the Pentateuch, the language would have been closer to Ugaritic or the Canaanite found in the Tell el-Amarna tablets -- somewhat like Chaucer compared to our modern English. Rather, the language is much closer to the Hebrew of 1-2 Samuel (in the case of the some of the patriarchical narratives) or Jeremiah or Ezekiel (in the case of the material assigned to D and P, respectively). The poetry of Exodus 15 is clearly more archaic than that of the prose, but it is still later than the putative era of "Moses" (as the poem looks back on the "conquest" of Canaan as a past event). In fact, the author of the prose frame into which the poem is embedded misconstrued the yiqtol preterite tense forms in the passages in Exodus 15 referring to the "conquest", interpreting them as referring to the future (thereby allowing him to put the poem on the lips of Moses prior to the conquest). This is because the use of this inflected form for the preterite, found at Ugarit and in the Tell el-Amarna tablets, died out in classical biblical Hebrew, where yiqtol came to only be used with imperfect and jussive meaning. The preterite use of wayyiqtol is even later (a form that was the result of grammaticalization), and this is the form that predominates in P, such as in the creation account in Genesis 1. This same evidence persuades me against the view that the Pentuateuch was substantially written in the Hellenistic era, as the language does not seem to be that late (i.e. roughly contemporaneous with Daniel and some of the Hebrew Dead Sea Scroll compositions), although it is clear that the redactions (even substantive ones) continued well into that period and later. I think the notion of a rolling corpus (as seen in Isaiah, Ezekiel, Jeremiah, Zechariah, 1 Enoch, Mathew, etc.) with respect to the Pentateuch may better explain the history of the work than the classic model of the Documentary Hypothesis. I recall seeing the website of a doctoral student (I think) who was pursuing such an analysis, but I didn't take down the URL.
The methodology of determining the sources of the Pentateuch is not much different than that found throughout literary criticism in assessing the integrity and authorship of various works, whether in Greco-Roman classics, the early Jewish/Christian pseudopigrapha, the early church fathers, the Pseudo-Clementine literature, Shakespeare, etc. In another thread, I listed out the many criteria and features that co-occur in P and set off this material as distinct in the Pentateuch, but I cannot find that thread right now. One may similarly compare that with, say, the stylistic criteria listed in Milton Brown's 1964 JBL article on the style of Pseudo-Ignatius, the later hand who interpolated material and whole epistles into the corpus of the epistles of Ignatius.