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You could probably analyze the tune in your car and look for certain signatures of a likely modified tune. Things like torque control turned off, cooling fan temps changed, etc. I used to use HPtuners, but switched to EFIlive. Unfortunately the original stock tune to my C5Z is long gone in the transition over the years.
As for mechanically being stock, yeah, it can be hard to say. Seems that some people will return a drivetrain to stock configuration before selling it. But normally we are only talking about very minor mods, and probably not something indicative of the engine being abused. The LS6 is a pretty robust engine, it seems, so even with some spirited driving in it's past, it should still be fine.
First thing I would do is to change out all the fluids. That will give you a baseline of knowing that at least all of that is fresh as of now.
Hello from Fort Lauderdale
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