Iv always believed and saw a few sources that only legit Pokemon have that checker. And fake Pokemon don't. Isn't this true?
The blue pentagon means that the Pokemon was hatched/caught in Kalos. If your Pokemon does not have a blue pentagon, try putting it in the GTS. If you can, it's legit. If not, it's fake.
The Blue Pentagon indicates the Pokémon to be caught in Kalos. If there is no Pentagon, it can either mean you transferred it from Gen 5, or it was hacked.