If a Pok?mon is hacked in and was made to look like a legitimate Pok?mon, there really is no way to know for sure if it is hacked even if it seems too good to be true. Every now and then though, someone will give the Pok?mon an incorrect detail where you can tell that they are hacked by yourself... given you know what to look for. Certain Pok?mon can't be in certain ball types, certain Pok?mon can never be shiny, certain Pok?mon can never be found with their HA in a particular location. Sometimes it's a combination of the ball type with the ability/location, or the ability with the location, or the shininess with the location/ability.
A good example would be a Aromatisse with the following details; Love Ball, Route 7, Aroma Veil. Now this is hacked because the Love Ball is only available in HGSS and SM, and Aromatisse is available in neither of those games. Another reason why it is hacked, is because Spritzee can't spawn with Aroma Veil on Route 7 as its HA is only available in the Friend Safari. Additionally, prior to SM, Apricorn Balls and Hidden Abilities were mutually exclusive to each other (and they still are on some Pok?mon).