Can a person see the righteousness of another, along with their misdeeds and sins, simply by looking at their face, or is that a temptation from Satan?

Belief in the miracles (karamat) of the awliya' (saints/pious predecessors) is agreed upon by the people of Sunnah. Among these miracles is "insight" (basirah), through which Allah informs whomever He wills of His servants about people's true nature, either from their facial expressions or from the tone of their speech. This is known as "firasah" (physiognomy/discernment). Firasah is of two types: inspirational, which comes through inspiration from Allah Almighty, and acquired/learned, which involves inferring inner realities from outward appearances.

No one's claim to know the details of the unseen (ghayb) through firasah is acceptable, because Allah Almighty has not revealed the unseen to anyone except the Messengers. Furthermore, the Prophet, peace be upon him, did not know everything about the hypocrites in Medina. It is also characteristic of the miracles of the righteous awliya' that they conceal them and do not display them.