Isn't every person's Aikido different? Even within the same dojo? As it comes from that individual's particular abilities and combination of mind, body, approach toward training, experiences, mindsets etc.
If this is the case, then all we can do is judge Aikido at the moment in time that we are judging "someone's Aikido". The simple truth is that EVERYONE'S Aikido is different at some level, but the thing that makes "Aikido" different from something like Boxing for instance is a set of principles that define what each system is supposed to achieve.
Hence, if a person has a solid grounding and understanding of "the principles" (with Shodan being understood as someone who has a solid grounding in the basics in many places), then one who understands the principles should be able to determine for him/herself that (A) What they are seeing can be called Aikido (in principle), and (B) Whether it's any good (i.e. if the performer is adhering to the principles). Of course the principles that different groups may choose to focus on may be different, but that does not change the number of principles, only the ones that are focussed on.
I mean, do we need to see every possible permutation of kotegaeshi to be able to effectively understand and apply the principle of the technique? I hope not.
Peter gave the example of a 5th Dan who said that Peter could not judge his Aikido, but then if the principles that we adhere to as Aikidoka are the same, then why not?
If we do not adhere to the same principles, then maybe we are not doing Aikido as we had thought. The question then becomes, who determines what are the original core of Aikido's principles. I'm sure there is enough recorded history of Ueshiba M. in training to figure that one out though.
Just a few thoughts. When I first read this thread I began to wonder if Tomiki K. came across similar comments in his time.
Train hard people.