The reasonings are different for Christians and Jews, and I am not as familiar with the reasonings for Jews.
That being said, you must understand for Christians, the law was instituted not to give a guide on how to live, but to demonstrate the impossibility of attaining God's standard of righteousness by personal effort. It's there to show you if you are going to be saved, it's because of what God does for you, not what you do to satisfy God.
It's kind of like the old joke about "if you have to ask how much it costs, you can't afford it."
I was asking how you personally can distinguish between one of those laws that must be followed, and one of those laws that it's ok to break.