donnajayne, I bought one off ebay and it was genuine. I took it to a jewellers who deals with Tiffany. As long as it has the right engraving on it it should be ok. I have seen them for sale on ebay that are supposed to be genuine but then they aren't even engraved !! They are the ones to beware of !
real tiffany necklaces have a tiny silver tag as part of the chain (next to the catch) with 'tiffany' engraved in it. also it would originally have come in a turquoise box and bag with white ribbon - although it might not come with if off ebay.