The Saints give the body of Christ; the Catholic church, some clues.
St. Thomas Aquinas, Summa Theologica, Pt. I-II, Q. 103, A. 4: “All ceremonies are professions of faith, in which the interior worship of God consists. Now man can make profession of his inward faith, (BY DEEDS AS WELL AS BY WORDS): and in either profession, if he make a false declaration, he sins mortally.”
St. Robert Bellarmine, De Romano Pontifice, II, 30: “… for men are not bound, or able to read hearts; but when they (SEE) that someone is a heretic by (HIS EXTERNAL WORKS), they judge him to be a heretic pure and simple, and condemn him as a heretic.”
The Magisterium of the Catholic church has also given instructions to its laity, when they see heretic by word and deed. Its dogmatic because its taken from the infallible scriptures; Titus 3:10-11.
Titus 3:10-11 A man that is a heretic, after the first and second admonition, avoid: Knowing that he, that is such an one, is subverted, and sinneth, being condemned by his own judgment.