How do I ensure that my AirPods are genuine or fake?

Some counterfeit devices are close to real, some are not.

Serial numbers cannot be used to identify counterfeits. Counterfeiters routinely copy legitimate numbers.

https://arstechnica.com/gadgets/2023/11/sparse-innards-of-25-counterfeit-airpods-pro-revealed-by-ct-scans/

https://www.youtube.com/watch?v=Db99cXMD780 (Adam Savage of Tested and Mythbusters reviews counterfeit AirPods)

We cannot tell you that I am afraid, as the fakes can be really real and even have real serial numbers