How can you assess that the epic poem was memorized (correctly) if you don't have a written record of the original version (I'll assume where there's reading there's writing)? Maybe it wasn't even epic at first.

Actually, I vaguely remember reading about a study of bards which showed that -- surprise, surprise -- thousand-line epic poems aren't memorized word-for-word. Certain famous lines are memorized, the overall plot is memorized, the meter may or may not be fixed... but a bunch of the details end up being improvised. Fans of folk music or jazz should be completely unsurprised by this.

The moral of this story is that the identity and talent of the person "transcribing" an epic poem into print -- or playing that folk music into the recorder -- are really important.

Of course, perhaps a dedicated critic of writing would laugh at the absurd notion that an epic poem whose words were exactly the same at every performance was somehow "more correct". Why, if you standardize the words, the art is gone! That's the approach taken by dedicated fans of concert music and jazz: it's the little differences in interpretation and approach that make each performance fresh and unique.