If I am not supposed to judge others, how can I say that someone is sinning?
Often, it is very easy to recognize the sin in someone else's life that closely mirrors our own weaknesses. Jesus calls us to examine ourselves first. In Matthew 7:5, He is recorded as saying,"You hypocrite, first take the plank out of your own eye, and then you will be able to see clearly to remove the speck from your brother's eye."
We must recognize that we cannot "pass judgment" on another person. But, we can, and should, see sin for what it is and avoid it. If it is sin in the life of another believer, then we have a duty to approach that person, in love, regarding their sin.
I would caution, again, examine yourself first! Even if you have never committed the sin that you have seen in another person and plan to address, you have committed sin. You have received the same measure of forgiveness from your heavenly Father and are not, nor will ever be, "better" than the person you plan to correct.
If, after all of this honest examination, you still feel it necessary to confront the sin, then you most likely have been called on by God to do it. For God's sake (literally), do it humbly and with much prayer.
"If you have any encouragement from being united with Christ, if any comfort from his love, if any fellowship with the Spirit, if any tenderness and compassion, then make my joy complete by being like-minded; having the same love, being one in spirit and purpose. Do nothing out of selfish ambition or vain conceit, but in humility consider others better than yourselves" (Philippians 2:1-3).