There's no substitute for playing a whistle. Short of that, for a review here are a few things that would help me a lot:
1. Record a reel or jig that has a nice mix of
well-executed traditional ornaments in both octaves--rolls, cuts, slides. This would give some idea of responsiveness and what a well-played roll or cut sounds like on that whistle.
2. Record the same tune on a common cheap whistle, using the same recording equipment, holding the whistle the same distance from the mike, playing the tune the same way, do this either right before or after you play the reviewed whistle, so all factors are the same. This way, I can hear how it compares to a whistle that I know, so there's a base line to compare to. A recording of the new whistle by itself doesn't tell me much, because I don't know how the recording set-up and the player's style affected the recording. Having the same tune recorded the same way on a cheapie will tell me something like, "okay, this person sounds like that on a Generation, it sounds like this new whistle may be more responsive, rolls are a little more clear, it's a little less breathy in the second octave, maybe not as balanced volume-wise as the Generation, etc."
3. Scales and a slow tune on both whistles would be cool, too, to give an idea of how the volume compares, what the Cnat sounds like, etc.
These reviews are fun, but any more I'll only consider buying a whistle based on someone else's recommendation if I know from clips that the person giving the opinion is a
very good player who plays in a traditional style, and knows what they're talking about. Which eliminates just about everyone.
Since my favorite whistles are not Generation-type, and that's not the sound I'm shooting for either, I imagine that "meh, I guess it's alright, if you're willing to spend that kind of money for something that doesn't even sound like a proper whistle" is probably the best I can hope to shoot for. 
