Although hacking is seen to be as unfair by many, there’s always a way to get Pokemon (or other items), even though it seems impossible. We can thank (or curse) Action Replays for this. I’m just generalizing, since I don’t know how to hack, but one of my friends taught me about hacked Pokemon.
The first sign it is hacked is if it is shiny. Legends never appear in shiny form (Phione is maybe one of the only legends that can be legitimately shiny, because when you breed Manaphy, you will get it). The location can be different from where it was originally placed in the game. For example, Zekrom and Reshiram, depending on the game, were encountered at N’s Castle (or Dragonspiral Tower) at Level 50. Since you are given the freedom (I believe) to set the location and the level, one can distinguish this. However, imported Pokemon are a much tougher case, seeing as only the region of the game it came from is displayed.
Hacked Pokemon appear everywhere, from Battle Subway (the Wi-Fi subway) data to GTS negotiations. A friend of mine told me that there is no way to put a hacked Pokemon on the GTS, but I am sort of skeptical. After all, there are some ignorant people that want legends that are under Level 9. As of now, it is impossible to stop the spread of Pokemon that have been spawned from an external device like GameShark. People can also enter their hacked Pokemon into Wi-Fi battles, but only for show. Fortunately, there is no way to alter a Pokemon’s stats as to make them all unrealistic (i.e. 999 for each stat). That would catch on too easily, since max stats reach usually about 300-400.
We know that shiny Pokemon that are obtained legitimately are the crown jewel of a trainer’s Pokemon collection. With the corruption in the Pokemon World, players (most likely kids) are buying either GameSharks or Action Replays and spawning these rare shinies, making it impossible to determine if it was obtained legitimately. Remember, the odds in finding a shiny (including by breeding) are about 1 in 8,000, which is definitely a motive of creating shinies. What makes them rarer is that the chance stays constant.
For example, if you cross a wild Pokemon, there is a 1 in about an 8,000 chance it will be shiny. If you run into another one, the chance does not go down to 1 in 7,999; it stays. Perhaps another motive of hacking is to show off. My opinion is that a completed legitimate Pokedex should be what all trainers strive for. After all, we were all sent out to complete the Pokedex by Pokemon Professors, correct? And it is a very daunting task, trading from each version, going on Wi-Fi to fill in those gaps… But one day, hopefully, those who abuse Pokemon will learn that it’s not the one with the rarest Pokemon that wins, but the one who plays with the most heart.