How can I identify an authentic Apple 5W USB Power Adapter?

apart from buying it from an apple store or official reseller there is no way, fakers can put valid serial number and copy both the case and the device itself down to the tiniest details.

Perhaps more importantly is what, specifically, you are intending to charge?

A 5W Power Adapter may be unsuitable for many recent Apple devices - and for those devices that can use a 5W power source, charging will be very slow. To benefit from faster charging, many recent models of iPhone will benefit from use of a 20W USB-C Power Adapter that supports USB PD (Power Delivery).

If you can provide more information about the device (or devices) that you intend to use with this Power Adapter, contributors here will be able to offer appropriate advice or guidance.

FYI ... The original Apple 5 W USB Power Adapter was introduced in July 2008 alongside the iPhone 3G and remained in production and sale for well over a decade. It continued being bundled with iPhones through the iPhone 11, and Apple kept it available as a standalone accessory in some regions until around mid‑2022. As others have already mentioned, if you intend to use it with your iPhone 16 Pro Max, you will find it very limited for charging, if not, non-usable at all for that purpose.

Ref: Power adapters for iPhone - Apple Support

For what it's worth it appears to be authentic. The fact its seal has not been broken is also a good sign.

However... it is very old, and 5W is not sufficient to charge most recent Apple devices. For AirPods or similar low power devices, it's ok. Other than insufficient power using it won't harm anything.

What are you planning on using this for? It's not going to be very useful for your current phone.

+1

I only use mine for overnight charging a 14 pro max or to help maintain a charge on long flights.