I don't trust American duelists w/ Japanese decks, b/c if they start to lose, they'll lie about effects. Like I knew one guy who had japanese Magical Hats, and tried to tell me that if I hit one of the non-monster cards, it'd activate it. But that's not how it works.
I do trust Japanese duelists, b/c they usually (atleast the ones I duel) have enough skill and honor to not lie about cards. (And some say their tactics are partly the reason SORL are limited.)
But usually, I'd just ask them to use only recognizable Japanese cards (i.e the Magnet Warriors, SORL, GAF, HFD.)