The exact colors of the Pokemon card back were never changed since the designs inception, so you can test the authenticity of a Pokemon card by comparing its back design to an original one
This, of course, is hardly a deterrent and in many cases can actually benefit a player
Jester's Cap is still fringe-playable to this day
Performapal Celestial Magician X DARK Level 4 [ Spellcaster / PendulumEffect ] ATK 1500 DEF 1000 P Scale 8 When exactly 1 face-up Fusion, Synchro, or Xyz Monster you control that was Special Summoned from the Extra Deck is destroyed by battle, or an opponent's card effect: You can Special Summon that destroyed monster, then destroy this card
The mewtwo ex card itself, particularly the full-art version from Black and White: Next Destinies, crossed over strongly between competitive players and collectors, and high-grade copies continue to hold solid secondary market value