First up, I want to inform you that although I am a Christian. I am by no means a biblical scholar, at least I don’t think I’m one.  I do however believe that the Bible (I favor the KJV but I think the subsequent versions serve it’s purpose as well) is the word of God.  I do have personal insights on this topic though.

 You were posing the question (i paraphrase) of how Abraham, Isaac, and Israel knew they were pleasing to God without having any written Torah or writings at all to base their certainty on.  I think there are at least 2 possibilities.

1.  They knew whenever they stepped over the line because God would deal out punishment in one form or another

2.  From the days before they discovered and followed God, they remembered what the spiritual and sometimes physical sensation of guilt felt like, and therefore, if they never ever felt guilt like that again, then they knew they were pleasing God

3.  Or, and this might tie in to #2 is that their knowledge was internal.  I can give examples of my personal experiences, they may seem silly to you but they make a valid point.  When I was a child, sometimes my mother would buy candy and keep it in the refrigerator.  Most of the time, the candy (chocolate) was for her, and occasionally it was for me and my sister.  One morning, I quietly left my bedroomand proceeded to raid the refrigerator (I was 5) for this candy while my mother was asleep.  Just as I touched the wrapper, I felt a voice… I can’t really say I heard a voice because it was more like feeling but still using clear english words.  The words, from the best of my recollection, said "are you stealing?"  Then as I stopped to try to figure out why i was hearing something without hearing something, the internal-instinct-based-voice again said, "why are you stealing from your mother?"  My point to this childhood story is to remind people of the power of conscience AND the power of the Holy Spirit.  I think of the human conscience as the microphone for the Holy Spirit.  This analogy works when you consider the fact that some people seem to not have a conscience; in reality, they have their microphone turned off so the Holy Spirit can’t (I use the term can’t loosely, obviously God is all powerful and can do anything) speak to them with the same loudness and clarity.  It doesn’t mean he isn’t speaking, it just means his message is either or both harder to hear and harder to understand.  The switch for the microphone is switched to the on position whenever we seek "truth" and it is switched to the "off" position whenever we seek anything that is irrelevant or unrelated to truth.

I personally think #2 and #3 answer this paradox better.  Once Abraham had experienced God, he managed to keep his microphone switched to the "on" position all the time.  In addition, in Abraham’s day, distractions weren’t as common as the distractions we have today.  In an era without telephones, cars, televisions, and computers, radios, and a world where a person told time by looking at the sky, the distractions were considerably less.  Afterall, imagine how much quieter your life would be without all of the above mentioned luxuries?  God speaks all the time, but it tends to only be in quiet moments that we hear him the clearest.  Abraham and his sons had that advantage of long periods of silence.  Another thing to consider is after they discovered God, and through faith and understanding, recognized a universal truth, they probably clung to it for all their might.  If you’re lost in a very dark place, a place where you can’t see the hand in front of your face then you are scared/concerned/worried etc./ and then someone walks by holding one tiny little candle, no matter how small it is, you recognize that a tiny bit of light is better than trying to find your way out of the place in complete and utter darkness.  Once Abraham and his descendants saw the light, they wouldn’t let it go for anything, they clung to it tightly without letting go so that after the epiphenal moment, they were always close to God and therefore were never ever outside God’s will again.

 Wow, I’m not usually so long winded but I’m opinionated at times, and I feel strongly on this one.  I would love to hear your thoughts.