Did you rewrap it? If it didn't come with a green wrap and a black insulator ring, there is a good chance it is fake.
If you rewrapped it, if there was any seams in the old wrap, it is fake.
The bottom is awfully clean as well, if you didn't clean it, then there is the chance it's fake because normally there would be a little wear on the negative from the manufacturing process.
If you got it from ebay or amazon, there is a good chance it's fake. Anybody that feels like it can easily sell anything on either of these sites and there is no way to tell if they are honest sellers or not. Could even easily claim to be who they aren't, like claiming they work for a reputable battery dealer.