How to check if my Apple 20W adapter is original?
Apple adapter 20w check original
[Re-Titled by Moderator]
By purchasing from Apple or an authorized reseller and breaking the tamper-evident seals yourself.
Counterfeits can be cosmetically quite convincing while still being dangerous to you or your hardware. Serial numbers of legit adaptors can be mass-reproduced as well.
About Apple USB power adapters - Apple Support
See more about counterfeits here:
http://www.righto.com/2014/05/a-look-inside-ipad-chargers-pricey.html?m=1