The only for sure way to know if you have genuine AirPods is to purchase them from the Apple Store or Authorized Apple seller. If purchased anywhere else on the internet, then they will most likely be fake. The counterfeit market for Apple hardware is huge and they can provide fake serial numbers and documentation just as easily as the hardware to fool you into thinking they are legitimate. The fake serial numbers may appear legitimate when you look them up, but that does not mean that they are not fake, just that they copied the serial number. You may find these problems with them:
- Poor Audio Quality
- Noise Cancellation does not work
- Find My does not work
- No AppleCare warranty
Also be aware that when purchased second hand, you may get a pair that has been disabled with Activation Lock. There is no way to bypass this and you would never be able to use them.