"Judgment" has such a bad ring to it. Probably this is because it has been misused so often. We have so easily judged one another, oblivious to the fact that we have been guilty of the same (or worse) offenses. Jesus clearly taught us not to judge one another, for by the same standard that we judge others, we ourselves will be judged (Matthew 7:1,2). In his inimitable style, he taught that we should remove the beam from our own eye first, then we will be able to see more clearly to remove the speck from our brother's eye. This latter implies an attitude of discernment (the Greek word anakrino shares the same root and is sometimes translated as judgment) but the motivation is one of helpfulness, not condemnation. Note that Jesus ultimately does not teach us not to remove the speck from our brother's eye, only to remove the plank from our own eye first.
And that attitude is the pivot point. There are times, in scripture as in the spiritual life, that we are called upon to discern and to be helpful to one another. If someone is found in a sin, a spiritual brother should go to him in order to restore him (again, the goal is not condemnataion or punishment without a plan of reconciliation). Paul begged the question, as if it was widely understood, "Are you not to judge those inside the church?" (1 Corinthians 5:12), while in other places, he cautioned believers not to become petty and judgmental over disputable matters (Romans 14:4). As usual, these matters become complicated simply because they are NOT all or nothing, black or white. There is a time to share our judgments and discernments, and time not to. Figuring out when, why, and how is part of the group process of spiritual maturity. We can sin by judging, to be certain, just as we can sin by turning a blind eye to the sin of another. Sorting this out on our knees before the Lord is crucial, being careful lest we stumble ourselves. A basic rule of thumb for deciding to confront might be: "Is it helpful? Is it truthful? Is it necessary?" Consulting with other mature believers can bring perspective when the problems have become too emotionally charged for us.
We are to exercise judgment (discernment) in listening to what we are taught. These days, teaching comes at us in the forms of books, articles, blogs (that's right), music, commentary from the platform before and after the message, and the sermon proper. We are admonished to pay attention, and to run what we hear and see through the filter of scripture. This takes some maturity, but even a new believer can do this with some effort. In fact, the new convert is more likely to exert the effort. The complacent Christian may become lax and drift away from the discipline of guarding the pure and simpe truth. A healthy, spiritual leader will welcome your questions. Luke describes the believers at the church in Berea as more noble than the rest, for they searched the scriptures daily to see whether the things they were being taught were true(Acts 17:11).
As our lives become informed by scripture (truth), we develop a frame of reference that gives us judgment about our lives. Beyond the obvious choices of avoiding sins, there are issues of motivations, obedience to God, benefit to His Kingdom versus self. These kinds of values begin to inform our decisions and guide our choices. Our judgment becomes grounded in biblical wisdom. This happens only when the collection of thoughts and facts become integrated, and we develop integrity. I beleive this is what James is referring to when he says we should not be like the male (Greek aner) who looks in the mirror and immediately forgets what he sees; rather, we should look deep into God's word and let it inform all that we are and all that we do.
Then we will exercise judgment and discern wisely for our own sakes, and for the sakes of others. Our judgment will be grounded in grace because we will not forget our own need of it. Our motives will be empowered by love because His Spirit flows through us to do His will for the sake of His Kingdom.
"You hypcrite, first take the plank out of your own eye, and then you will see clearly to remove the speck from your brother's eye." Matthew 7:5
"Brothers, if someone is caught in a sin, you who are spiritual should restore him gently. But watch yourself or you also may be tempted." Galatians 6:1
"Are you not to judge those inside the church?" (1 Corinthians 5:12)