How do I know if my Hermes Birkin bag is a fake one?

When I was 8, my aunt gave me a bag and i had no idea what brand it was. Then I clicked on Eva Longoria's most expensive item, and it was the same bag, only made out of crocodile skin. Now I know it's probably Hermes Birkin, but how do I know if it really is Hermes, and if it's not fake?