Christ's admonishment, to judge not lest ye be judged, concerns the interior disposition of the soul.  We can't determine who is saved and who is damned because, unlike God, we can't know a person's conscience when he or she leaves this mortal life.  Was the sinner (each of us is one) perfectly contrite and remorseful of his or her sins?  Did this person die in a state of sanctifying grace?  We simply can't know for certain.
The trouble with the "who are we to judge" mentality as used in our times, is that it attempts to excuse sin and immoral behavior, as if we can't know the difference between virtue and vice and shouldn't judge behaviors accordingly.  There is a big difference between judging behavior and judging the interior disposition of the soul.  When it comes to the former, we must make those judgements, in fact we are directed to do so, else how can we pull our brother aside and admonish him in the spirit of true Charity? What are we to make of Matthew 10:14?