Luke 6:45

The good man produces what is good and honorable and moral out of the good treasure (stored) in his heart; and the evil man produces what is wicked and depraved out of the evil (in his heart); for his mouth speaks from the overflow of his heart.

What is in our heart, we speak. We may think we are hiding our feelings, but our feelings bubble to the surface and come out in our words. Even when we joke, we are still expressing our feelings and showing our true personality and what we are thinking. If we need a heart check, listen to what we speak, then change and repent of the thoughts.