Here are some parameters:
1- Eloquence and literary power: The Qur’an was revealed in 7th century Arabia. The highest form of art among Arabs was eloquence and poetry. The Qur’an challenged them to bring something as powerful as the Qur’an in its eloquence and literary power. They could not. Many attempts were made but they all failed.
2- The scientific references mentioned in the Qur’an. At the time, no one knew these scientific realities, and the Prophet (s) never did any scientific experiments. Who can come up with scientific facts without any experimentation and they hold true for over 1400 years? This is a challenge to humanity
3- The predictions the Qur’an made and all turned out true
In sum: the Qur’an says that humanity cannot produce a book like the Qur’an: it’s miraculous in its eloquence, phonetics, scientific detail, legal detail, historical detail of previous prophets and nations, it’s comprehensiveness in addressing all aspects of life, such as business law, family law, inheritance, social interaction, worship law, and so on.