How do you know if a Pokémon card is valuable?

How do you know if a Pokémon card is valuable?
Look in the bottom right corner of the card to find the rarity symbol, next to the card number:

A circle means the card is common, while a diamond marks uncommon cards.
A star means the card is rare, while a star H or three stars are special, extra-rare cards.

Look in the bottom right corner of the card to find the rarity symbol, next to the card number:
  1. A circle means the card is common, while a diamond marks uncommon cards.
  2. A star means the card is rare, while a star H or three stars are special, extra-rare cards.