I guess I would want to first assume that the question is referring to early Christian apologists because it is so easy for present apologists and even apologists of the most recent preceding centuries to determine which teachings are and are not Orthodox. When the Traditions of the Church were originally being formed by early Christian apologists and Fathers, though, I guess I would assume that they lived the faith in a way that we are no longer really able to. When you live the faith, you just know the faith. I think heresies are rooted in spiritual and mental infirmities, so people like Arios or Nestorios who were originally part of the true Christianity must have had this sort of infirmity. But early defenders of and apologists for the Faith were living the Faith, and knew God in the way Saint Paul said he wanted to know Him.
