Serial number inside the case

You should see your AirPod’s serial number inside the case on the inner side of the lid. If there’s nothing there, your AirPods aren’t genuine.
Don’t worry too much about whether the number is on your case’s left or right side. A Reddit post tells us that there is no official consensus within Apple on where it’s printed. Apparently, the OP contacted Apple Support, and they were unable to give a definitive response on what side you should find it on. According to his research, the Reddit blogger concludes that it may be printed on either side depending on where they were created and the region in which you bought them.
Once you’ve checked for the presence of a serial number inside your charging case, it’s a good idea to match that with the one in your box. Sometimes, scammers will give you an original box but swap the product inside for a fake one. You should have nothing to worry about if both numbers are exactly the same.