And Catholics know that how?
Formally, the teaching authority in the Church is vested in the local bishop or more precisely the ordinary (some belong to orders that do not follow the territorial jurisdiction). One can also rely onthe publications sanctioned by the Church, e.g. consiliar documents and the Catechism. With the Internet, it is easy for anyone to educate himself, and I urge even the non-Catholics to read on what the Catholic Church proposes for their salvation.
This stands in stark contrast with the Protestant sects, where the poor Protestant can never know who to trust for instruction, and the truth is, he should trust no one in any of the Prtestant communities, as neitehr of thme is lead by the Holy Ghost.