Frank Menetrez in "Elena Kagan’s Harvard: Golden Age or Reign of Error?" states that Norman Finkelstein was right to accuse Alan Dershowitz of plagiarism from Joan Peters. One "smoking gun" is identical errors that Dershowitz makes. Menetrez states:
I looked at one of the passages identified by Finkelstein, a long quotation from Mark Twain, and found that Dershowitz’s version of the quotation and the version in the book Dershowitz was accused of plagiarizing [from Peters] contained 20 identical errors in a mere 21 lines of text. Some of the errors were large (such as the omission of 87 pages of text without an ellipsis) and some were small (such as altered or missing words or punctuation), but the cumulative weight of the evidence was overwhelming. There was no way Dershowitz could have independently generated exactly those 20 errors -- he must have copied them. It was an open-and-shut case.
Here are the relevant PDFs of Dershowitz, Peters and Twain's writings so people can indepently verify Menetrez's accusation.