Even a child makes himself known by his acts,
by whether his conduct is pure and upright.
How do we "make known" what is in our hearts? How do we reveal our faith? By our behavior.
But one can deceive through his actions. One can pretend to be pure and upright. Not a child. He invariably gives himself away even if he wants to deceive. And not even an adult, not if he is observed closely. He will give himself away as well, for he is unable to constantly keep up the pretense. Those whom he fools are those not paying attention.
The bottom line of this proverb, as with other scriptures on the same theme, is that the condition of the heart is revealed through how one lives. When we protest, "What you see is not really me," we are deceiving ourselves. The burst of anger is not produced by outside causes; it springs up from within. As Jesus said, "For from within, out of the heart of man, come evil thoughts," etc., etc. (cf Mark 7:14-23).
It is fine to work on conduct, but we will make the most progress by being keen observers of our conduct and then looking into our hearts. Behavior is my best diagnostic tool for knowing the heart. Don't go by feeling. Go by what is observable.