There are a surprising number of hidden values / details with which a naive hacker could be tripped up - especially for event Pokemon - but it is definitely pos

There are a surprising number of hidden values / details with which a naive hacker could be tripped up - especially for event Pokemon - but it is definitely possible to create undetectable hacked Pokemon with enough work. There are also cloning glitches in many of the games that allow one perfect Pokemon to be replicated and distributed to multiple people, although that's mostly used for casual play because it'd be suspicious for multiple competitors to show up at a tournament with exactly identical Pokemon.