On the curious question of how Christians know heresy from orthodoxy, I found a great quote from St. Irenaeus of Lyons in his book Against the Heresies:
"Anyone who keeps unswervingly in himself the canon of truth received through baptism will recognize the names and sayings and parables from the Scriptures, but this blasphemous hypothesis of theirs [viz. of the heretics] he will not recognize. For if he recognizes the jewels, he will not accept the fox as the image of the king. He will restore each one of the passages to its proper order, and having fit it into the body of the truth, he will lay bare their fabrication and show that it is without support." (via: http://pleroma.typepad.com/)
It is interesting that St. Irenaeus does not point us in this instance, to a magisterium, but rather to the Gospel, which is signified in baptism (one of the ordinary means of grace).