Hi Profwag,
Well, that's the million dollar question
isn't it? Unfortunately, this is a mechanistic approach to something that really doesn't fit in that shoebox very well. In real life, this ability is simply blended in with the other senses so seamlessly to the point that even for psychics it is difficult to separate them. An acquaintance of mine refers to this as the "total readout." I suspect that this is why the ability shows so poorly in testing. It is unnatural to isolate it. It also improves with knowledge, which is admittedly frustrating for anyone who is trying to separate this skill out from a knowledgeable guess, but the fact is, the more you know about a subject, the better your psi will be in that area. I can look at people and get a sense for where they're hurting, but a doctor with a similar level of psychic ability would probably be able to give a rather detailed diagnosis. In fact, there are some anecdotal stories of this. I think in Russia they even have some medical psychics to help in diagnosis.
But I digress. You would need to test the general population for psychic ability and have a fairly substantial database to even begin to answer this question in any quantified way. This will give you numbers, but it still doesn't necessarily speak to who has the best psi. There seems to be a bit of a trade off of emotional stability for psychic ability so that when you're asking people to do a very stable activity, such as testing, the best won't respond very well. Testing is loaded with pressure, it is repetitive and stressful and isn't very interesting nor is there any emotional attachment. These things work against strong manifestations of psi.
As psychics evaluating psychics, we do what we do best; go by feel. Since we are so good at empathy, we look at how people are feeling. A person being psychic is quiet inside and a fraud is excessively noisy. A psychic has to turn inward and a fraud turns outward. A person well trained in body language could probably spot the difference, it's that obvious when you know what to look for.