So then what is your system for determining literal and non-literal?
Excellent question. The problem, of course, is it assumes that such distinctions are native to the literature in question. There is a real possibility that we are doing harm to the nature of the genre of Genesis by asking to be either "literal" or "non-literal."
Even in English literature, like the terms "fiction" and "non-fiction," they are blunt tools to describe some of the best books going around. The problem arises when they are treated as mutually exclusive. However, they aren't. The Wikipedia definitions are instructive in this regard.
Fiction: any form of narrative which deals, in part or in whole, with events that are not factual, but rather, imaginary and invented by its author(s).
Non-fiction: an account, narrative, or representation of a subject which an author presents as fact
http://en.wikipedia.org/wiki/Fact.
For example,
Moby Dick. Originally it was believed to be invented; now the consensus supposes it to be generally factual. But, like Genesis, the absence of critical interpretative instructions frustrate certainty.
Thus, as with
Moby Dick, we are left with a story -- in this case a powerful story with a message about humankind and in particular its relationship to the deity.
The important thing to realize is the weight of this ambiguity. It should force the reader to avoid asking questions that turn on whether Adam and Eve "really" existed, or if there "really" was a talking snake, or a woman made from a rib, etc.
The validity of the story doesn't depend on its factuality or fictionality. The question of historicity is a distraction from what the narrative is trying to teach us about ourselves, about our world and about our God.