While some people would feel safe by verifying the barcode, it can still be complicated when it comes to identifying an original Xiaomi charger. Why? Because even the fake can be imitated when it comes to barcode. As for some people, they may rely on the heaviness, which may not be totally bad. But you know what? Fake can be imitated by using an iron that makes it weigh heavier.
This is why, based on what I've come to notice (and still observing), if you want to be close to 'I am sure it's the original', pay close attention to these 2 things; 1, sharpness of the font (REMINDER: not just the type, but sharpness) 2, colour code (hint: the orange colour of the originals are usually light, not deep). Meanwhile, as for the best method, in case you still find it hard to differentiate, is to ask a vendor who's reliable.
Reliable is that he or she has deep knowledge about that product + willing to let you know if it's original and if it's fake. Do you get it?