BIGGEST way to tell a fake from an authentic juicy couture bag, on the pink label inside the bag, it will say 'Love G&P', NOT 'Love P&G'!!! all the fakes I have seen have 'Love P&G' and this is the first thing I check when purchasing a bag along with the other tips mentioned by others.
Also, thought id mention it wont always have a white sticker with 9 numbers inside the pink label - I bought 2 of my bags from the Juicy store at Mall of Millenia, Orlando, and the other from Macys, at the Florida Mall, Orlando and the one from Macys has the sticker, but the other 2 from the Juicy store do not.
I have 3 authentic Juicy bags and was caught out on EBAY buy a seller promising me the bag I was purchasing was authentic and she assured me she had a reciept which she would send with the bag, but she didnt include it with the bag, I paid ALOT of money for this bag, but as soon as it arrived I knew instantly it was a fake! Dont be duped into paying Juicy money for a poor copy made in China!