"Who is wise? - He who learns from every man" (Avot 4:1)

A wise man recognizes himself and his shortcomings, whereas a fool knows others enough to criticize and disparage them but does not come to know himself, to be aware of his own shortcomings. Yet every man has the ability to know himself when he wishes to. As Rabbeinu Tam wrote in his Sefer HaYashar, this is a main fundamental principle, and an important rule.

Every person can know himself and his spiritual level….

Now, it is written in the Zohar (II 251a-b, also 6b) that in the realm of holiness there is a hall of innocence and virtue, and conversely, in the outer realm of evil there is a hall of guilt. The difference between the two is that in the hall of innocence and virtue, when a man is judged there, all present arguments search to find him innocent and meritorious - even at the judgment of a wicked person. On the other hand, in the hall of guilt, all present arguments to show that the person on trial is guilty and should be punished.

Hence, in this way every person can know himself and his spiritual level. If he judges every man favorably, to find him good and virtuous, he is in a state of holiness, because in that realm is the hall of innocence and virtue, so called because there they argue for the merit and goodness of everyone. But if he judges everyone to be bad and guilty, then he is on the outer realm of evil, since there the hall of guilt is found, where a man is always judged in a bad light.

In this connection I quoted elsewhere, "Do not judge your fellowman until you arrive at his situation."  (Avot 2:4)

[Kutonet Passim, parashat Acharei Mot]