Your best bet in sourcing legitimate product is to always purchase from authorized retailers, such as your local bicycle dealer. We are unable to authenticate SRAM products with the serial number defaced or missing.
An authorized/legitimate retailer would always offer after sale service support, the product would come with all parts required, and the serial numbers would be present. This is something to look for when purchasing from online retailers in the future.
See our product authentication notice here.
If you need help finding the serial number on your component, see our serial number locator here.