How can you tell what lives in a person's heart?

Our Heavenly Father and the Lord Jesus are very wise.  If you want to know true wisdom, you have to spend time with the Holy Spirit.  He is the truth revealer.  He will give us all of the wisdom that we need.  The Lord Jesus provided an answer to our question of "How can you tell what lives in a person's heart?".  It is recorded in Matthew 12:33-34, which says, "33 “Either make the tree good and its fruit good, or else make the tree bad and its fruit bad; for a tree is known by its fruit. 34 Brood of vipers! How can you, being evil, speak good things? For out of the abundance of the heart the mouth speaks.”"  So if we are trying to evaluate what's in the heart of a person, we can first evaluate what dominates their conversation with you.  What someone has in abundance in their heart is going to come out.  Secondly, we can observe their behavior.  You see the behavior of a person is the true revealer of that person.