Philology and historical sources.
See e.g. T.H. Irvine, The Platonic Corpus, into Gail Fine (editor) The Oxford Handbook of Plato (2008, Oxford University Press):
six works [are] listed under "spurious" [De Iusto, De Virtute, Demodocus, Sisyphus, Eryxias, and Axiochus]. In addition to the six recognized spurious works, other works are "dubious," regarded as spurious by most or many modern students. Some of these dubious works are philosophically significant, including the Hippias Major, Clitopho, Theages, and Alcibiades. Of these, the first two are probably genuine, and the last two are probably spurious. No one has argued that any of the six acknowledged spurious works is authentic.
Thirteen letters are included that purport to be by Plato. Most modern students reject almost all the letters as spurious. The most important letter about which serious dispute remains is Letter VII. If this letter is genuine, it is important, for two reasons: (a) It offers some autobiographical detail about Plato. (b) It puts forward philosophical claims that have no parallel in the dialogues. Probably, however, Letter VII is spurious and is not a reliable source for Plato’s life or for his philosophy.
See also the SEP's entry on Plato.