The Holy Spirit, without whom I would not be Catholic (nor many other things, like chaste). If I weren’t Catholic, I would believe much differently than I do. The second most influential thing was being raised in the Christian faith. By that I mean the people around me, what I did, what opportunities I had, what I was told was true, the witness of the saints.
If you mean currently, when confronted with the opportunity to believe something that I don’t, what do I do? Like, say I didn’t know that Christ was present whole under each species. If I were suddenly confronted with this, I would most likely try to confirm it by an ecumenical council. It might be a good idea to try looking in sacred scripture about that topic, but I would be likely to think I already knew the Eucharistic passages therein, so I probably would not look, at least, not right away. Once I found it in an ecumenical council, then I easily might want to re-read those passages with a new eye. Also, I must point out that the easiest way to find out where such a thing is and in which council is to consult a theology text (or now that the CCC exists, it might have it) and check out its footnotes. Oh, and to find out which page to check footnotes on, I’d use the index. I love books with nice indices.
BTW, I am aware of the meaning of 1 Cor 11. I’m ignoring that for the sake of guessing what I would do. Also, I probably would not look in an ecumenical council first if the topic were embryonic stem cells.