Most importantly, THE NUMBERS STAMPED ON GM PARTS ARE NOT PART NUMBERS. They are CASTING numbers and as-such, are not credible for reference. Using wheel center caps as an example, I've got four Cadillac wheel center caps; all have the same number on the back and all are different. Oh, they will all interchange in-terms of fitment - they are all the same size and made to the same fitment specifications. But they differ in finish (chrome vs. painted vs. satin) and also in logo design.

So, how to find out the PART NUMBER for a GM part? Well, the easiest way, is when you have a new part; you can just look at the part number on the box.
Because some of the parts that I distribute are either used or 'take-off', there isn't a box to pull a part number from. You may be in a similar situation, attempting to replace a part that was factory-installed. Or worse, you may have a part that was put into a different box (re-boxed) and the number on the box isn't correct. This can be especially frustrating, so always look at a previously-opened GM parts box with some skepticism.

There are two ways to find the GM part number for a part you seek.
The better way, is to take the last 8 digits of your vehicle's VIN to your dealer's parts counter, and together figure out the part. You see, GM maintains a database of every single part that your vehicle originally came equipped with. So, armed with the last 8 digits of your (GM) vehicle's VIN, the dealer can tell not only the exact options on your particular GM vehicle, but the actual part number of the part you seek. Be careful - in the case of wheels - if you have different wheels than your vehicle was originally-equipped with, this obviously won't work.

The other way to figure out the GM part number you seek, is going to make GM parts counters cringe when they read this. Yes, it IS possible to decipher the GM part number, from that GM CASTING number that is actually stamped on it. It's just going to take ALOT of work, including a phone call to someone at GM Engineering. Again, while it can be done, consider this a method of last resort.

In my case, I just ask people to e-mail me a picture of the part they seek. Specifically, if it's a wheel or a center cap, I ask for pictures of both and can always tell them because this is my specialty.

Hope this helps!