How can one identify a deceiver, and is a person who swears by God to be believed despite the questioner not being convinced by his words and possessing evidence of his falsehood, knowing that the deceiver follows the same methodology as the questioner?

The default assumption regarding Muslims is their uprightness, and one must have good thoughts about them, for Allah Almighty says: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin." And the Prophet, peace and blessings be upon him, said: "Beware of suspicion, for suspicion is the falsest of speech."

Nevertheless, a Muslim must be cautious and not fall for the deception of deceivers, for Allah Almighty says: "O you who have believed, take your precaution." And 'Umar ibn al-Khattab, may Allah be pleased with him, said: "I am not a deceiver, nor can a deceiver deceive me."

One who practices deception should be advised to fear Allah and be made aware that deception and lying are unlawful, for the Prophet, peace and blessings be upon him, said: "Treachery, deception, and betrayal are in the Fire," and "He who deceives us is not of us, and treachery and deception are in the Fire." One must beware of him and warn Muslims about him if he does not repent.