I can answer part of that question - at least for a XLR, if it was used by GM for a show car (or perhaps photo ops/commercials), it (1) has a code for that on the build sheet, & (2) the original invoice will show it was delivered to GM instead of a dealer. If you get the "view vehicle build" part of the service records, it will have the code "ZR6 - SHOW VEHICLE." It's on the build sheet so the plant knows to do their best work when assembling it, if it is to be a show car (as opposed to being test driven by the press). There's nothing different about the VIN, however.
Learned all of this from our dealer when we got the V (used), & it is backed up by the paperwork that the car was built & used as some sort of GM show car. It was shipped to auction with 40 miles on it when GM was done, which apparently is the typical mileage. I don't know what all gets done differently when the build sheet has a show car code, but people have told us the paint looks like it has an extra layer of clear coat. I had no idea how manufacturers did any of that so it was fascinating to learn!
We also bought a STS that was used in a local new car show (mini-local downscale SEMA). It was originally delivered to the dealer, then the dealer transferred title back to GM for purposes of putting it in the show, & when GM was done title was transferred back to the dealer. That weird series of transactions meant we could buy it as a new car OR as a CPO. I don't think there's any way you could find out something like that happened unless the dealer told you - Carfax would just show a title change, not who it went to. I didn't even mention that when I sold it because there was no documentation of it. (by the way, getting a show car that way is not as cool as it sounds, as it was wiped down nonstop at the show so the clear coat was completely scratched up & trashed!)
I also know that when GM has "V Performance Academies" where you can go learn about & track their car, & tear it up instead of your own, they refurbish the car & sell it to a GM employee. Or at least that is what they told us when we asked. I've seen those cars for sale by dealers later & they come with NO power train warranty no matter age or miles.
My guess would be a similar thing happens with cars the media gets to drive, i.e., if the drive is set up through GM instead of a dealer, the invoice probably shows the car was delivered to GM and then the car is transferred to a GM employee, executive or otherwise. That's the only way you might know the car was "whipped by a journalist," as opposed to just being whipped by a prior owner (or a dealership employee)!
