How do we tell if someone is a believer or not? Is it simply a matter of their profession? In general, we look for more than this. In this passage, Jesus indicates that what we should be looking for is fruit in the lives of those who profess Christ. The Pharisees claimed to be righteous men but Jesus indicates (especially after their blasphemy of the Spirit) that they were producing bad fruit and this called their claims into question.
So we look for good deeds and good words in those who claim to be Christ’s. But there must be even more than this - there must be motivation for these things that is grounded in love for God and a desire to obey and glorify Him.
Jesus then turns to look at the tongue. We see how easy it is to use the tongue for wrong purposes and we are challenged to be very careful how we use our own tongues, since we must give account for every word they utter. We should use them to build up and not to tear down, to build unity rather than division.