I am reposting here my answer (which ended up as comments) to a question on Yahoo!Answers on how to tell the difference between real and fake Havaianas flip flops.
1. Check the sides of the slippers. Fake Havaianas have small holes left by air bubbles. Real Havaianas are more compact.
2. Real Havaianas are heavier than fakes.
3. Straps of fakes are made of plastic and do not bend easily unlike the real ones.
4. Check the underside, real ones have rectangular marks that are pronounced (for traction, i guess). I forgot what was the design of the fake ones but they were not as pronounced.
5. Check the footbed, the fake ones’ oval marks are more pronounced than the real ones.
6. For fake ones, the stamp on the footbed says “made in USA”.
7. Real Havaianas Highs have soles.
And as Turreau has also mentioned in the page, better check the official websites to see if the designs and styles are there.