There are quite few ways to tell or identify fake or original Power Bank
It has no Brand Name: Some power banks in the market has no brand name, instead what is written on them is just "Power Bank". Don't go for them! There are many branded ones you can checkout for. Example are those from Xiaomi, Anker, Jackberry, iMuto, iceworks, MaxOak, Qi-infinity and so on.
Exaggerated Ratings: Many power banks that are rated above 30,000mAh capacity are usually fake except those from popular brands.
Lite Weight: Most fake Power banks weighs very lite. You will see a power bank with 50,000mAh capacity weighing very lite and even fancy looking and easy to put inside pockets. My dear, it's fake. An original 50,000mAh supposed to be decently heavy and bulky.
They are Very Cheap: Most fake power banks are very cheap, in fact, their prices are too good to be true. You will see a seller pressuring you to buy 30,000mAh capacity powerbank for just N3,000.