We are to Judge Angels.

2 Or do you not know that the saints will judge the world? And if the world is to be judged by you, are you incompetent to try trivial cases?
3 Do you not know that we are to judge angels? How much more, then, matters pertaining to this life!
4 So if you have such cases, why do you lay them before those who have no standing in the church?  (1 Corinthians 5 – 4 – ESV)

According to this verse we are one day to judge Angels. We also should be able to judge matters of this earth. But somehow the world, and many Christians have gotten this mixed up.
First it isn’t judging that is forbidden it is hypocritical judging that is forbidden.
Can you imagine a world where no one judges anyone for anything?
We could not lock up murderers or rapist, because we would have to judge them as guilty first. All of our laws depend on the ability to judge if a person is guilty or not.

The trick is how and when are we to judge?
We are to judge with love, if not for the love of the person being judged, with love for the people that person will hurt in the future if he isn’t locked up.
It would not be very loving to let a serial killer free, to kill again.
We are not to judge hypocritically. A few years ago there was a preacher that was bashing homosexuals on a regular basis. It was later discovered that he had used the services of a male prostitute.  That was an example of hypocritical judging, the kind of judging that Matt 7:1 forbids.
We are not to go crazy with our judging either. Nit picking people to death will drive people further away from Christ.
Whenever possible just talk to the person, and in the least judgmental way possible tell them why you think what that person is wrong , and let that person decide for themselves if they need to change their behavior or not.