According to the YouTuber Austin John, there are three surefire ways to identify a Ditto without fighting it. The first method is to hold down the ZL button, which lets you focus on your nearest target. If you have, say, a Flaffy registered in your Pokédex and use ZL to look at it, the game should tell you that it’s a Flaffy and what level it is. If its name shows up as “???” instead, then it’s probably a Ditto.
Another method is to pay close attention to a Pokémon’s behavior. In Pokémon Legends: Arceus, different species of Pokémon react to players in different ways. These creatures might run away, observe players, or try to scratch their faces off. Pokémon Scarlet and Violet adopt this mechanic, and it’s a great way to identify Ditto since, while they can look like any Pokémon, they don’t necessarily act like their disguises. If you encounter a Pokémon that is supposed to be aggressive but isn’t, odds are you’ve actually found a Ditto. Another YouTuber, Aero, documented that this change in behavior can extend to a Pokémon’s movement. For instance, Jigglypuff are supposed to bounce around like the balloons they are, but a Ditto disguised as one will only waddle.
Now before I dive into the third possible Dittot identification method, there’s something you should know. Previously, I said if a Pokémon is acting weird or its name isn’t showing up properly, then you’ve only probably found a Ditto. That’s because you might have found a Zorua instead. Like Ditto, a Zorua transforms at the start of combat, and their names show up as “???” while you’re holding ZL. So how do you tell the difference between Ditto and Zorua without fighting? Well, Ditto are content to observe trainers, while Zorua are invariably skittish and will run away.
As such, the third and final way to uncover Ditto is to essentially cheat by saving the game, closing it, and then rebooting the program. According to Twitter user Atrius, Pokémon Scarlet and Violet remember the Pokémon closest to players when logging out, but if any of them are Ditto (or Zorua), the game doesn’t remember their disguises. If you save the game, close it, and log back in, any Ditto will show up as their normal, amorphous selves in the overworld. Hey, one of the bugs in this game had to work out in your favor.
With this knowledge in hand, hunting Ditto should be much easier.