When Nintendo introduced Pokemon to the world in 1996, the series took off with a roar, resulting in more than just video games: It spawned anime and a trading card game that remains popular almost 20 years later. Not only does the trading card game still hold favor in gamers' hearts, certain cards found in the deck retain high value. There are plenty of things you want to keep in mind when trying to tell if a Pokemon card is rare or valuable.
Check the rarity symbol. Every Pokemon card has a symbol in the bottom right corner. Common cards have circles, which mean they're not worth anything. Diamond cards are uncommon, and they can go either way in value. Stars are the rarest cards, and some can go for high prices.
Check the portrait of the Pokemon. If the Pokemon portrait appears to have foil and shines in the light, it's a holographic card, which makes it considerably more valuable than its basic brethren. Only rare cards are holographic; you'll never find a diamond or a circle card with foil on it. Starred but basic cards may still be valuable, but holographic cards are normally the ones that climb in price.
Look for a first-edition mark, located beneath the Pokemon portrait. If the mark is there, the value of the card is higher. As time goes on, chances increase that people will have reprints of cards, making it less likely that one has a first-edition, making these more valuable to collectors.