In-game, exile is usually your best bet
Here, we see an Ultra Rare version of the Mega-Evolution of Chandelure, showing the Pokmon outlined in shimmering gold while a second, pink and blue shadowed form is floating in the background
Once it arrives, claiming it is a different card is a 'probatio diabolica' (proving a negative), so it is already too late
If your opponents Active Pokmon is an Evolved Pokmon, devolve it by putting the highest Stage Evolution card on it into your opponents hand
If all players can't agree on what's in the art, you may consult an outside person for a final determination
Regardless of which camp you are in, sit back, grab your beverage of choice and relaxwe are going to take a trip back in time to the very beginning of Pokmon