By their fruit you will recognize them.
Do people pick grapes from thornbushes, or figs from thistles?
Likewise every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire.
Thus, by their fruit you will recognize them.
As I mentioned in the previous post, which was ages ago, this is teaching on who we are. And I would like you to notice that the focus of this passage is recognition of others. And even though this passage could be used to do some introspection, it is primarily for discerning who the "bad trees" are.
I know that in this modern society we live in, we "aren't allowed" to talk about anyone else, but the Bible would clearly say differently. We are commanded to discern (judge) who the people around us are, from the teachers that we listen to, on down to the friends that influence us on a day to day basis.
There is absolutely nothing wrong with going to someone and saying, "I love ya. And I love ya enough to tell you that your actions are not those of a Christian." In fact, not only is there nothing wrong with this, there is everything right with this, if it is done in Love!
Do you have people in your life who are claiming to be Christians, but aren't living that way? If so, don't be afraid of confronting them.