How to find out if the former owner of my Pokemon game has cheated?

[kaoskat]

I have no idea honestly.

[Einhornbaby]

Thank you all for your advise
We now finally have a Poké-Bank account and transferred many Pokémon to it (not all yet, we are working on it). Thankfully we only lost a small handful of them (those non-legit ones that could not be transferred). We are through the used copies of Pokémon Black Edition and Pokémon White Edition we own, but didnt try to trade those from Diamond and Perl etc. yet.