I'm a event collector (or i was) and to tell the truth, like 80 % of the pokemon from events here in PC are hacks.
Using pokesav, ar codes or others programs to alter games, hackers can have a nearly same pokemon from event.
Only expert with truly knowledge and a great experience, and using specified programs, can tell you if these pokes are legit or not.
Obviously if you downloaded via mystery gift, they're surely legit, if you trade them from another trainer, i recommend you have all of them checked ;D