Ok, I'm going to give up on the word natural for now. It is getting us into a lot of trouble.
I think we have to try and understand what we are talking about before we go any further though.
What is a way to "prove" you have internal power? If internal power is just subjective to peoples opinions it might not be a real thing at all.
What would you offer up as a proof of internal power? And could you find a video demonstration of it? And if this video demonstration can be duplicated is that proof that the duplicator has internal power, or would the person duplicating still have to meet the approval of others before we deemed him internally powerful?
I would not rely on video from it, because if the person in it has skill, it will only be visible to someone else who has some level of that same skill. To anyone else, it may look staged, or they probably can't tell the difference anyways.
The best way is firsthand experience. I had the opportunity to touch Mike, and despite the fact that I don't have much of anything of these skills, I could recognize similarities between him and Akuzawa when I tried to push him. Even if you don't have any skill, it is readily apparent
when you push against someone who doesn't have it and someone who does. Further, it feels just completely different when that same person strikes you, grabs you etc. (There is a reason I fly to japan every couple months besides having random Korean girls buy me drinks and sleeping on Rob's floor
The second best way is to rely upon the testimony from others who have felt it or others recognized to have it to some degree. We have plenty of those people around here on Aikiweb.
I must stress, the best possible way is to feel it first hand, and hopefully glean someting from it. there is undoubtably different levels of it, and for me at this time is is more about consistancy than anything else.
As for "how to" you can follow some of the exercises Rob posted, but I think it is really hard to get the "intent" without someone else there to help you, though I have a feeling that overall stability may improve (for example learning to do ashiage).