I got a wurmple in a master ball once, I was suspicious. Then again, maybe he was conned by his friend's "use the master ball on something stupid" trick lol
As far as that goes, I find the GTS pretty good for trades, as long as you ask for the normal things (like Shelmet-Karrablast, or Haunter-Haunter) rather than powerful Pokémon. I have no idea, maybe I'll get them checked someday, but they all look at least 90% legit. I still have that inkling of doubt that maayyyyyybe he's a Pokémon hacking piece of ****. yeah
Then again, I don't plan on really "competing" where people will see my Pokémon's stats, so RNG is the way to go for the trades around this game.