“As a man thinks in his heart, so he is”
— Proverbs 23:7
Opportunists who are out to get whatever they can for themselves often talk a good game. The way to catch them out is to watch their interactions with other people, especially with people who don’t have anything to offer them. Jesus understood this:
Oh generation of vipers, how can you, being evil, say good things? For the mouth speaks out of the abundance of the heart. A good man, out of the good treasured in his heart, brings forth good things; and an evil man out of the evil he treasures, brings forth evil things.
Jesus offers this warning to the disingenuous:
Every idle word that men speak, they will have to account for on the day of judgement. By your words you will be justified, and by your words you will be condemned.