How do we distinguish between the whisperings (waswas) that are considered a sign of true faith and those that may be a result of weak faith?

The self-doubt (waswasah) that a Muslim finds too grave to utter is from the clear signs of faith, because the Muslim's deeming it grave and their intense fear of articulating it is proof of the perfection of faith and the absence of doubts. Thus, deeming these whisperings (waswas) grave and repelling them indicates the soundness of faith. As for pursuing and spreading these whisperings, it indicates weakness of faith, and their doer is feared to fall into shirk (polytheism). And whoever repels waswas, it will be repelled from him, as Allah Almighty says: "Indeed, the plot of Satan has ever been weak." [An-Nisa: 76].