There is a woman I know of who is a very harsh judge of character.  She prides herself on being able to look at a situation and declare whether or not the parties involved are Christian or not.  Whenever she heard about someone in her circle of family or friends who was pregnant but not married, or who got in trouble with the law, or engaged in some other form of sin, she immediately condemned them.  It never sat well with me.  I often listened to her, assessments and thought to myself, "but you have made some questionable choices."

Whenever I am about to declare someone a good or bad christian, I stop.  I ask myself three questions: Have you ever messed up? Does God hold it against you? Do you really have the RIGHT to judge this person?  The answers are usually: Daily, not if I repent, and absolutely not.
I don't always get it right.  But I usually stop myself from calling a situation something that it may not be.  God knows all, not me! 

This is not to say, that if you see your brother or sister doing wrong you let it slide.  Our job is to imitate Christ.  Remember the woman who was about to be stoned for adultery in John chapter 8?  Jesus did not judge her.  He forgave her and encouraged her to do right.  He didn't shake his finger at the Pharisees.  He told them, "You better check yourself before you throw that stone."  (Scripture says that he wrote in the sand....I think he started listed the names of those with stones in their hands who also committed adultery.)

The next time you find yourself about to call someone on their sin, make sure you take a moment to look in the mirror.  If you must approach the person...do it in love.