For one, the United States tried to discourage Al-Jazeera from showing the tape. Another clue, did Al Qaeda deny the authenticity of the video? If not, then that is another hint the tape was not a fake.
Before the tape was aired, the State Department asked the government of Qatar to discourage Al-Jazeera from broadcasting it, a senior State Department official said.
In other words, the U.S. government did not want the tape aired.
That admission was made on militant websites by "the general leadership" of al Qaeda.