Enlightenment from between the hands of the Torment

The Messenger, between the hands of the Divine torment, is a Divine mercy. So how can this Mercy be distinguished? How the Messenger can be known among many false calls? How the truth can be known?!
How did the Muslims distinguish Muhammad sawas and followed him without Musaylimah or Sajjaj or Al-Aswad Al-Ansi and other Christian scholars, or Jewish scholars or Hanafi scholars?