(condensed from Ein Ayah, Berachot 9:223)

Gemara: Rava said: A person knows about himself whether he is a tzaddik (righteous person) or a rasha (evil person).

Ein Ayah: Admittedly, we have said that a person’s nature has great influence, to the extent that it determines how he will analyze matters and decide whether certain actions are morally good or bad. Therefore, we would think that a rasha would not be able to judge himself because he is so surrounded by his wickedness that he thinks that his ways are good ones.

However, the aforementioned self-deception exists only in regard to matters of deciding what to do. In that regard, the tzaddik believes in that which is good, the rasha believes that it is good to do that which is actually bad, and the average person will see things in a non-consistent way. Yet there is a general understanding that every person has, which is not impacted by his characteristics and actions. The problem of the rasha is that this understanding does not connect properly to the specific action that is before a person; rather he forgets the moral truth when needed at the opportune time. Internally, though, even the rasha possesses a feeling of straightness and truthfulness that his Maker endowed him with. This sense is able to discern that which is absolute good, and, with it, he is able to see whether he is good or evil.