There doesn’t seem to be a way to know what Dex number a monster is, without either asking someone, or checking it if you already own it (in which case you don’t need the dex).
I wish there was a way we could filter like in Create your team.
If you don’t know the number it takes ages to find something. But sometimes we want to check the move set of monsters we don’t own. Buff the monster dex search option.