How can one distinguish between authentic and fabricated chains of transmission (isnads) in hadith, especially since some innovators (Ahl al-Bid'ah) claim their narrations have isnads?

The standards of Hadith sciences are applied to any transmitted report, regardless of its transmitters. Scholars of al-Jarh wa al-Ta'deel (discrediting and accrediting) judge the state of a narrator by presenting his narrations to reliable narrators and comparing them to the narrations of his peers, in order to assess his precision and ascertain his uprightness through the testimony of his contemporaries. The Ahl al-Sunnah stipulate a continuous chain of narration from its first narrator to its last. Others, however, rely on their beliefs to judge narrations and narrators; they accept what is narrated by those they believe to be infallible, and some of them have even disparaged the majority of the Companions. As for the Ahl al-Sunnah, they uphold justice in judging narrators and their narrations, authenticating and weakening based on the narrator's state and precision. Al-Shaykhan (Bukhari and Muslim) and other narrators from the Ahl al-Sunnah have narrated from individuals affiliated with opposing sects such as the Shi'a, and others from the Khawarij, Nawasib, Qadariyyah, and Murji'ah, according to specific criteria. The Ahl al-Sunnah have been meticulous in critiquing narrations since the era of the Tabi'een, and documentation began early, while others were delayed in this regard.