I'm not avoiding THE ISSUE:
How do Catholics decide which TEACHING to follow?
That is a great question!
Jesus told the Apostles to go forth and teach what He had taught them. And so they did. As these teachings were recorded and saved unto history, that is one way.
The other way is when Jesus tells the Apostles they have the power to bind and loose; when they are told to “...go to the Church”; when St. Paul visits Peter. In all of these things, authority is shown and exercised. This authority didn’t end with Peter.
Additionally, the Apostles had the authority to appoint a successor to Judas, namely Matthias.
As you see, the Church has possessed the authority handed to it by Christ Himself.