"...so, every healthy tree bears good fruit, but the deceased tree bears bad fruit. A healthy tree cannot bear bad fruit, nor can a deceased tree bear good fruit."
Healthy tree? Deceased tree? Bad and good fruit? WHA?!
A healthy tree is a Christian. A person who has been born-again. A deceased [dead] tree is a person who has not turned to Christ.
While not judging others (you can read about that here), we can look at a persons actions to see if they are Christians. If you see a person keep repeating a sin, you can talk to them about it. But, don't be all like, "You are such a horrible person. Here, let me tell you why."
But if that person doesn't seem to think that he/she is sinning, or his/her words are not lined up with the Bible, you can guess that that person has not been born again. Then...
You can help them come to Christ.