How do I differentiate between acts of disbelief and the whispers of disbelief?

The nullifiers of apostasy are clear in Fatwa no. 146893. The default is that a Muslim remains Muslim until his departure from Islam is proven with certainty. So, whoever commits a nullifier with certainty and has the proof established against him, his disbelief is ruled. As for one whose status is doubted, the default is that he remains Muslim. Fleeting thoughts and ideas that do not settle in the heart do not necessitate apostasy; rather, they are whispers that are forgiven. Hating these whispers and striving to get rid of them is a sign of true faith and merits reward. The person afflicted with whispers should not pay attention to them and should not indulge in them.