How do the Hadith scholars determine who is the most reliable among narrators, and is that done through their shaykhs?

The precision of a narrator and their status in mastery are determined by comparing their narrations with those of reliable, masterly narrators. If their narrations largely agree with theirs, even if only in meaning, this indicates their precision, and rare discrepancies do not harm. However, if discrepancies are numerous and agreements are rare, then the instability of their precision is recognized, and their hadith is not accepted as proof. This methodology has been affirmed by An-Nawawi, As-Suyuti, and Ibn As-Salah, and other scholars such as Ash-Shafi'i, Ayyub As-Sakhtiyani, Ibn Al-Mubarak, and Imam Muslim, who emphasized that knowing a narrator's error or distinguishing the objectionable in their hadith is done by presenting their narration alongside the narrations of reliable, حافظ (memorizing) scholars.