That's a bit naive, don't you think?
People who know how to "hack" (set the memory addresses to values that in turn are represented by a pokemon with a name, 4 moves, gender, nature, etc) often know what is impossible to obtain in a cartridge.
And more often than not, hacking a pokemon directly will never be checked. If you're worried about tourney-worthy pokemon, hack two 31/31/31/31/31/31 parents and 1-step egghatch and just brute-force a seemingly legit pokemon.
Hopefully this answered the "hack-checker" question. Just make sure all the constituents of a pokemon are legit and the pokemon is in turn legit. No 999 stats, no impossible movesets, a sensible catch or hatch location, all the things we assume.