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Since the data structure for Gen 5 Pokémon has been completely reverse engineered, and everything can be faked, a 100% sure checker for legit Pokémon is impossible, legal is the best that can possibly be done.
Since this is more of a quick question, it doesn't really warrant its own thread. If you have any more questions regarding Pokémon X/Y and/or Pokémon Bank, then make sure to use the Quick Questions thread.