How did scholars distinguish the trustworthy from the weak, and how can we do that today?

The Hadith (definition) scholars would verify the status of a narrator by examining their integrity (adalah), righteousness, uprightness, and the precision of their memorization or writing of Hadith. Their method for assessing a narrator's memorization and mastery involved comparing their Hadith narrations with those of their contemporaries and with their own narrations at different times and under various circumstances. They would not authenticate a narrator solely based on their apparent righteousness; rather, they relied on their conduct in narrating Hadith.

As for the modern era, knowing who is trustworthy (thiqat) and upright (udool) is no longer required in the same manner as it once was, because this is not an era of narration. Instead, the Muslims in this era primarily rely on compiled Hadith books. The integrity (adalah) required in our time is integrity in religious matters: meaning adherence to obligations and avoidance of prohibitions, ensuring that a Muslim is not committing a major sin, nor persistently committing a minor sin, and is not foolish in intellect. This encompasses righteousness in religion and chivalry (muru'ah).