Is there any way of knowing or finding out if cologne or perfume is authentic or not?

i've seen lots of places that sell colognes and perfumes at amazing prices(sometimes almost half of what you would pay at a department store.) these look identical to the original and smell very similar..is there any sure way of knowing for sure? or is the deal really that good?Is there any way of knowing or finding out if cologne or perfume is authentic or not?
there is no way of finding out if it is authenic or not. only how anybody could tell is if you tell them. i love the cheaper version of curious. look on the bright side you will smell great and save money! :)

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