visually in photos, its hard to tell. But the following physical tests can be done:
* Check EEE codes (serial numbers on Apple’s GSX database).
* Check if duck head alignment peg is metal or plastic (it should be grounded, so needs to be metal).
* Check LED delay when plugged in. (the IC should check if charger is genuine when first plugged into a Mac. if the LED isn’t delayed by a second before it goes green, its not got the correct IC chip).
* Check IC chip ID and serial number (in system information panel of Mac).
* Check power output and curve with an oscilloscope. (compare with Apple original, as we do. cheap ones will fluctuate a lot and will have a lot of ‘noise’)
* Crack open the charger (or a few from a batch if ordering in bulk) and inspect build quality, and look for properly grounding.
Obviously these tests are not all viable for an individual consumer buying a single charger, although you could do a few of the basic checks. This is the sort of testing that is required of a supplier in the Apple market, such as us, when checking for original chargers.
You can tell, it just takes a lot of experience and hands on tests to tell for sure.
Spelling or grammatical errors are quite common on really bad knock offs.