I would say that one of the difficulties is that salvation is a word that different people use to mean different things. That being the case, I'm not sure I even understand what you mean by "judge someone's salvation." However, I think that one important thing for us to remember is that we need to believe in Jesus Christ and repent of our sins in order to enjoy the happiness that is available in Christ and through Christ. And again, I don't think I understand what you mean by "judge someone's salvation," but I suspect that a lot of people are missing the happiness that they might otherwise receive if they would focus more on helping others to believe in Christ and turn away from sin than on speaking words of condemnation against those who do believe in Christ but whose faith is not strong enough to keep them from a particular sin.
In any event, I would say that it is good for us to remember that we should not continue in sin, and also that we should encourage faith and repentance in all people, even those who are improperly "judging someone's salvation."