Judge not according to the appearance, but judge righteous judgment.
We are commanded both directly and indirectly throughout the scriptures to judge "righteous judgment". The Savior's direction to "Give not that which is holy unto the dogs, neither cast ye your pearls before swine", "Beware false prophets, ye shall know them by their fruits" and "Succor those that stand in need of succor" all require that we make righteous judgments of our fellow men. In order to effectively use our moral agency we must make judgments between that which is good and that which is not... between those we can trust in and those we cannot... those we should follow and those we should not. Sometimes a righteous judgment can be as simple as determining those who may need our help.
One talk which I have found particularly illuminating on this subject is, "Judge not and Judging" by Dallin H. Oakes. I recommend it highly.
In the 10th chapter of Luke we read the account of Martha as she hosted the Savior and her sister Mary for dinner. She labored in the kitchen while Mary sat at the Savior's feet receiving of His goodness. Placing yourself in Martha's position, can you not just see yourself thinking, "Don't mind me in here. I'm just trying to get dinner together so that I can sit down and enjoy the Savior's company too."? I certainly can! Really, it wasn't the most considerate thing for Mary to leave her sister to finish preparations on her own. In Matthew 18:15 the Savior teaches us the way to handle these types of situations... which simply are a reality of the human experience. He says,
"Moreover, if thy brother shall trespass against thee, go and tell him his fault between thee and him alone..."
When we take the Savior's counsel and openly address our concerns with and only with the person involved, we give the offender the opportunity to see and recognize that they have caused a problem (which they may or may not have realized) ... and we give ourselves the opportunity to understand another point of view. This often leads to our own realization that no offense was intended and was possibly even imagined!
When, on the other hand, we sit and fester, our judgment typically grows in harshness.
Picking up on the story...
40- But Martha was cumbered about much serving, and came to him, and said, Lord, dost thou not care that my sister hath left me to serve alone? bid her therefore that she help me.
41-And Jesus answered and said unto her, Martha, Martha, thou art careful and troubled about many things:
Note that the Savior acknowledges her concern, before delivering a clear yet gentle rebuke...
42- But one thing is needful: and Mary hath chosen that good part, which shall not be taken away from her.
To me it is significant that the Lord offered no correction of Martha's priorities until she complained to the Savior about her sister's priorities.
... with what judgment ye judge, ye shall be judged: and with what measure ye mete, it shall be measured to you again.
I have become very mindful of the fact that I set the standard for how the Lord will judge me by how I choose to judge others. If I spend my time criticizing others, either openly or in secret, I am in essence inviting the Lord's criticism of me. If I condemn those around me... I invite the Lord's condemnation. If on the other hand, I extend mercy and compassion to those around me... not only do I feel greater internal peace, but I invite the Lord's mercy and compassion on me in my weak and flawed effort.
An indispensable addition to this truth is that when we stand before the judgment bar of God, we will be judged according to "the word, that which I have spoken..." John 12:48 Our Father and elder brother, Jesus Christ, are the only two who can truly know what portion of the word we had the opportunity to either receive or reject in this mortal probation. Consequently they are the only two who have the right to administer final judgment. We should never venture to take this responsibility upon ourselves.
What a blessing it is to view those with whom we associate through the lens of kindness and compassion!