So what about the Ten Commandments? Which are they? That's a good question, and like most good questions, it doesn't have a simple answer. "No other gods," "Eschew any idols," "Don't misuse the Name," and "Honor your parents" are eternal principles. You could, I suppose, make the case that "Do not murder" and "Do not steal" are applications of the principles "People are made in God's image" and "Property rights are important." Based on Paul, I'd strongly make the case that "Keep the Sabbath Day holy" was a time-bound application of some greater principles.
Actually, based on the words of our Savior and Paul, you could make the case that "Love God with all your heart, mind, soul, and strength" and "Love your neighbor as yourself" are the bedrock principles, and everything else in the Torah is just application of them. I might have to think this through in more detail.
But with these exceptions aside, I think it's pretty clear most of the time what's an eternal principle vs. what's a time-bound application of a principle. If you read something in the Torah and you can't apply it directly today (e.g. you don't own a field and gleaning isn't an issue) or if the N.T. specifically says it doesn't directly apply to you any more (e.g. the dietary laws), then it seems to me that it's pretty clear it's an application. If it's repeated in the N.T. (e.g. "Love your neighbor as yourself"), then it's a principle you need to follow. Prayerfully ask yourself questions like "Why would God want his people in that time to do this? What does it say about his priorities, what he values highly and doesn't value highly?" If you don't come up with any good answers, a good study Bible or Bible teacher can probably help you out.