True to their signature gimmicks, Zorua and Ditto like to disguise as different Pokémon in the overworld. One way to tell them apart from the rest of the species they are imitating is to closely examine the species' behavior and find one that's acting rather odd and out of character. A faster way is to use the games' new lock-on feature as the Pokémon's name on the lock-on's UI will display as "Zorua" or "Ditto" (or as "???" if you have not yet encountered either of the imitating Pokémon).