Do prophets ascertain that it is God who is revealing to them every time He reveals to them?

It is impossible for revelation to the Prophet to be confused with Satan, for everything Allah reveals to the Prophet is definitively and certainly from Allah.

Revelation occurs in three forms: 1. Inspiration into the Prophet's heart: The Prophet does not doubt that what has been cast into his heart is from Allah. This includes the righteous dreams of the Prophets. 2. Allah's direct communication with His Messengers from behind a veil: As Allah spoke to Moses and Muhammad (peace be upon him). 3. Revelation through an angel: This is understood from the Almighty's saying: "Or He sends a messenger to reveal, by His permission, what He wills," (Qur'an 42:51) which usually refers to Jibreel.

These forms demonstrate that the Prophet cannot doubt that what comes to him is revelation from Allah. And if the Prophet makes an independent judgment concerning an matter about which no revelation has come to him, then revelation corrects him if he errs or confirms him if he is correct.