Sometimes we hear someone who proclaims to be a Christian criticize another person for doing something that doesn't seem right. Another Christian may say to that person, "How can we judge them? We don't know what it is in their heart. Only GOD knows."
First of all. We are to criticize. But not judge. Of course there is a difference. Only GOD can judge. But while it is true that we cannot measure a person's heart. We certainly fall witness to their life. The idea is that they are setting the proper example to help lead others out of the world. If their actions appear no better than those of a non-believer, then we should be skeptical of them.
When you see a celebrity standing on a stage, accepting a trophy for "Entertainer of the Year", it is safe to say that they are not walking in the Spirit of GOD. Because if they were, they would denounce their own fame and leave that life. So yes. A Spirit led life will be reflected for others to witness. If the image of our life contradicts what we are supposed to be, then we are NOT what we are supposed to be. This is the same thing as a parent understanding that they cannot tell their kids not to do something that they themselves are still guilty of doing. Hypocrisy.
Now, also understand that yes. We stumble. But we should not fall. We will not walk in perfect unison with Jesus Christ. We are like little children holding His hand. We can still stumble around. But His hand will keep us from actually falling if we keep holding it. Do not let go.