It is possible you could tell an event form an original by the pokeball the pokemon is in, because normally event pokemon are in cherish ball, it’s impossible to get a cherish ball through regular game play without cheating. On the other hand a raikou that is caught in a game would be in a regular pokeball or a masterball.
Also some event pokemon have special ribbons that cannot be obtained through regular gameplay or even by cheating, I don’t know if event raikou came with ribbons though.
This isn’t a sure fire strategy but it is another way to judge.
Hope this helps, sorry if it doesn’t