For Pokémon captured in Gen 4 and onwards, you can use this code in Batch editor:
Every event Pokémon Gen 3 Mew & Gen 3 Deoxys & Gen 4 and onwards (including VC transfers) must have the FateFul Encounter Flag, and this codes removes mons that doesn't have.
This way it'll be easier to see which are event mons. (Exception is Wild Event Darkrai)
As for Gen 3 event mons, you gotta go through the box to check.
Presumably I'll check known species, OTs and TID as potential candidates.
(However I think that Gen 3 non-island Mew/Deoxys events being added are pretty unlikely, as most were shiny locked)