Instructions
Examine the rarity indicator in the bottom right corner of the card. A black circle indicates a common card, a black diamond indicates an uncommon card, and a black star indicates a rare card. Some cards of popular evolved Pokemon or legendary Pokemon, such as Mewtwo and Charizard, are always rare.
Look for a promo star on the card. It may be in the bottom right corner, or below and to the right of the artwork. It is a black star with the word "PROMO" written in white on top.
Search the artwork for foils that indicate the cards origins. Some rare promotional cards were released only at certain events such as Comic Con, or for Pokemon Organized Play (POP). Those cards are marked with the event's foil rather than a generic promo star. These may be printed on the artwork itself. Any card marked "Prerelease" in foil on the lower right hand side of its artwork is a rare card.