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I'm guessing that you're looking at either a used C6 or C7. If it's a C6, post a link to this topic on the C6 Scan & Tune forum. Those guys will know for sure how to check out the ECU to see if it's stock.
If you're interested in a used C7, check with the dealership to find out if they have scanned the ECU. If it's not a GM dealership, ask if you can have the ECU scanned at a Chevy dealership (on your dime).