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There are several ways. Do you have a key now? If so, you can use a multi meter set on ohms and place one lead on each side of the resistor contacts to get the value. The picture below gives the 15 different key blank resistor values.
The other way would go to your local Chevy Dealership with the car's VIN and they should be able to look it up. You'll need to have some sort of document that identifies the car by the VIN and also list you as the owner. Something like a car's registration.'