In Story of a Soul, St. Therese of Lisieux offers wise and helpful words on this subject, that may perhaps help clear up some of the muddy waters we encounter in dealing with the details of everyday life.
Yes, I feel it, when I am charitable, it is Jesus alone who is acting in me, and the more united I am to Him, the more also do I love my Sisters. When I wish to increase this love in me, and when especially the devil tries to place before the eyes of my soul the faults of such and such a Sister who is less attractive to me, I hasten to search out her virtues, her good intentions; I tell myself that even if I did see her fall once, she could easily have won a great number of victories which she is hiding through humility, and that even what appears to me as a fault can very easily be an act of virtue because of her intentions. I have no trouble in convincing myself of this truth because of a little experience I had which showed me we must never judge.
During recreation the portress rang twice; the large workman's gate had to be opened to bring in some trees for the crib. Recreation was not too gay because you were no there, dear Mother (Therese is writing to her own blood sister, who is also her superior in Carmel), and I thought that if they sent me to serve as third party (the religious who accompanied the Procuratrix when laborers had to work in the monastery) I would be happy; at exactly that moment Mother Subprioress told me to go and serve in this capacity, or else the Sister who was at my side. Immediately I began to untie our apron but slowly in order that my companion untie hers before me, for I thought of giving her the pleasure of serving as third party. The Sister who was replace the Procuratrix was looking at us, and seeing me get up last, she said: "Ah! I thought as much that you were not going to gain this pearl for your crown, you were going too slowly."
Certainly, the whole community believed I had acted through selfishness, and I cannot say how much good such a small thing did to my soul, making me indulgent toward the weaknesses of others. This incident prevents me from being vain when I am judged favorably because I say to myself: Since one can take my little acts of virtue for imperfections, once can also be mistaken in taking for virtue what is nothing but imperfection. Then I say with St. Paul: 'To me if is a very small thing to be judged by you, of my any human tribunal, by neither do I judge myself. He who judges me is THE LORD.'
In order that this judgement be favorable or rather I be not judged at all, I want to be charitable in my thoughts towards others at all times, for Jesus has said: 'Judge not and you shall not be judged.'
St. Therese had ample opportunities to practice this form of humility, as indeed we all do. She saw these opportunities as gifts of God, and rarely if ever wasted them. Thus, when she was placed in a position of authority over the novices in the last years of her life, which required her to study them, seek out their imperfections, and help them improve, she was able to clearly see the imperfections without being blinded by any regard for herself or any attachment to her own ideas. Because of the time she faithfully dedicated to prayer and the practice of virtue, she saw everything through the lens of God.
Our judgement then, should rather be called a search for Truth, where we neither attempt to see into the heart of a person, nor allow ourselves to be shaken in our search.