Well if Ed Parker started American Kenpo, then it follows that any Kenpo being taught by his direct successors would be the closest to "authentic" American Kenpo than others.
So, to research: which among Ed Parkers black belts were the ones he considered the best, and who are their direct descendants?
Perhaps to answer that question, if Ed never actually named favorites, then just research which of those black belts attained the highest ranks eventually.
Now if you're looking at Kenpo other than American, I have no clue how to answer that.
I think that as long as you go into a Parker school that has direct lineage to a relatively high ranked 1st generation black belt then you will be pretty close to learning the right stuff. As to what direction he wanted to take and what is closest to that now, well I think that is open to conjecture. Whether the direction was SL4 such as the Doc teaches or Kenpo 5.0 such as Jeff Speakman teaches or the direction any of the others are teaching well that is a matter of opinion. Each high ranking black belt seems to have their own version of how closely they worked with Ed Parker. I'm not here to insult anyone I'm just saying very few of us were around Ed Parker at all so the only way we are able to establish who did what and how things were is by what we are told by those who were there. Each person seems to have a slightly different take on where they stood and the direction Ed Parker wanted to take. I have only read Doc's comments here and have met Jeff Speakman once and can say their versions are not identical as to how things were so as I said if there is direct lineage to a relatively high ranking 1st generation black belt then you are on the right track as far as choosing someone to train with goes. If however you want to just find out about where people stood and just get a gist as to how things were then the most qualified person to ask her would be Doc. But it would be wise to ask others who were there as well if you can track them down just to get a broad range of information