How do you know how rare a "Magic The Gathering" trading card is?

Well, you could search for them on Ebay and see how much people are bidding on them.

Look at the expansion symbol at the right side of the line between the text and artwork.

Black is common, silver is uncommon, and gold is rare. Legendary cards will state their legendary status on the text on the left of that line; Legendary artifact, Legendary Goblin….

What @ragingloli said.
That’s what always annoyed me about the game. The makers create an artificial rarity and drive up the price. You buy more decks so you can find those “rare” cards. That’s why I never played.

Meant to add to previous answer but ran out of lunch time…

@ragingloli The price of a card is not really based on rarity. Some “Rare” cards are worth less than popular “Uncommon” cards.

Note that each 15-card booster has 1 Rare, 4 Uncommons, and 10 Commons. Rares tend to be powerful; contrary to @Rarebear‘s cynical remark, the rarity is to make it difficult to create uber-decks (in other words, to balance the game) ,and certain formats ban Rares (and sometimes Uncommons). The makers only get paid for the boosters; they don’t get a share of the single-card sales, and most who construct decks with many rares buy singles, so there’s actually no financial incentive for them to make certain cards rare.

@jerv You’re right. I“m being unfair. I just don’t like being sucked into games like that. I like to pay for my games up front and that’s it.

@Rarebear If you want to go after the people who run the secondary market, you have my blessings. Some of their prices are a bit outrageous! That’s why I’m more pen-and-paper; I get a 300+ hardcover book printed in full color on glossy paper for less than what dealers charge for many single cards.