St Vincent of Lerins answered it:
How to Distinguish the True Faith from Heresy
At the moment when many Catholics are making, or considering making, compromises with Progressivism regarding the New Mass and Vatican II, it seems to us opportune to remember the criteria to maintain the true Catholic Faith given by St. Vincent de Lerins.
St. Vincent de Lerins
In the 5th century, St. Vincent of Lerins saw that the people were faced with various errors and heresies of Donatus, Arius, Photinus, Pelagius and others, and gave them this good advice on how they could know with security the true Catholic Faith. Even if it is taught by distinguished men or Prelates, the bad doctrine should not be accepted by Catholics, who should cling to Tradition and what has been believed everywhere, always, and by all [quod ubique, quod semper, quod ab omnibus creditum est]. Actually, he stated: “I have continually given the greatest pains and diligence to inquiring, from the greatest possible number of men outstanding in holiness and in doctrine, how I can secure a type of fixed and, as it were, general, guiding principle for distinguishing the true Catholic Faith from the degraded falsehoods of heresy.
“And the answer that I receive is always to this effect: That if I wish, or indeed if anyone wishes, to detect the deceits of heretics that arise and to avoid their snares and to keep healthy and sound in a robust faith, we ought, with the Lord’s help, to fortify our faith in a twofold manner, first, that is, by the authority of God’s Law, then, by the tradition of the Catholic Church.
“Here, it may be, someone will ask: ‘Since the canon of Scripture is complete, and is in itself abundantly sufficient, what need is there to join to it the interpretation of the Church?’ The answer is that because of the profundity itself of Scripture, all men do not place the same interpretation upon it. The statements of the same writer are explained by different men in different ways, so much so that it seems almost possible to extract from it as many opinions as there are men. Novatian expounds in one way, Sabellius in another, Donatus in another, Arius, Eunomius and Macedonius in another, Photinus, Apollinaris and Priscillian in another, Jovinian, Pelagius and Caelestius in another, and latterly Nestorius in another. Therefore, because of the intricacies of error, which is so multiform, there is great need for the laying down of a rule for the exposition of Prophets and Apostles in accordance with the standard of the interpretation of the Catholic Church.
“Now in the Catholic Church itself we take the greatest care to hold that which has been believed everywhere, always and by all. That is truly and properly ‘Catholic,’ as is shown by the very force and meaning of the word, which comprehends everything almost universally. We shall hold to this rule if we follow universality, antiquity, and consent. We shall follow universality if we acknowledge that one Faith to be true which the whole Church throughout the world confesses; antiquity if we in no wise depart from those interpretations which it is clear that our ancestors and fathers proclaimed; consent, if in antiquity itself, we keep following the definitions and opinions of all, or certainly nearly all, Bishops and Doctors alike.
“What then will the Catholic Christian do, if a small part of the Church has cut itself off from the communion of the universal Faith? The answer is sure. He will prefer the healthiness of the whole body to the morbid and corrupt limb.
“But what if some novel contagions try to infect the whole Church, and not merely a tiny part of it? Then he will take care to cleave to antiquity, which cannot now be led astray by any deceit of novelty.
“What if in antiquity itself two or three men, or it may be a city, or even a whole province be detected in error? Then he will take the greatest care to prefer the decrees of the ancient General Councils, if there are such, to the irresponsible ignorance of a few men.
“But what if some error arises regarding which nothing of this sort is to be found? Then he must do his best to compare the opinions of the Fathers and inquire their meaning, provided always that, though they belonged to diverse times and places, they yet continued in the faith and communion of the one Catholic Church; and let them be teachers approved and outstanding. And whatever he shall find to have been held, approved and taught, not by one or two only but by all equally and with one consent, openly, frequently, and persistently, let him take this as to be held by him without the slightest hesitation.”
(The Vincentian Canon, in Commonitorium, chap IV, 434, ed. Moxon, Cambridge Patristic Texts) http://www.traditioninaction.org/rel…serveFaith.htm