If the sticker has faded or fallen off, you will need to go under the dash on the passenger side and remove the kick panel to locate the PCM. The code should be on a sticker on the PCM harness. It is a 4 character code usually 3 letters and 1 number.
How to identify GM PCM?
https://www.youtube.com/watch?v=ozr4IQyFf0M
What is a GM ECM?
The Electronic Control Module -- or ECM, as it is more commonly referred to -- is a built-in computer in a vehicle that works by monitoring the associated components. Typically located underneath of the coolant reservoir, the engine ECM in a Chevy vehicle monitors emissions, fuel levels and temperatures in the vehicle.