In addition to the answer previously given, another thing to consider is that although the Qur'an references certain events in the Prophet's time, it speaks in a universal manner that is accessible to any reader, regardless of their society or era. It is very difficult for a human author, especially one who is not from an area with diversity and a high level of education available, to do this.
Also, when you consider the way that the Qur'anic texts unfolds, the style of speech is not the same that a human would use when organising a book; for instance, the conception of time is more overarching, as it would be from the divine perspective.
Some people also consider the presence of scientific or numerical miracles to be convincing evidence.
Ultimately this is something one can also look at the text and get a sense of!