I assume this is at a dealer?
Not much you can do to verify what the DME has calibrated without taking it to someone with dealer tools unless you have the VF programming and cable. Even then I am unsure if the VF stuff allows you to read the tune or not. You might want to see if the dealer can contact the previous owner and see if you can get their contact info to ask some of these questions.
Personally, I wouldn't want a car that's already been supercharged.