On a fake, the copyright dates on the bottom of the card will be blurry. On authentic cards, each word, number, character is sharp. Hold the cards up to a light. If you can see light through the card, it's fake. Fake ones are slightly larger than the real ones.
Real cards have richer, darker colors.
The best way to keep counterfeit cards out of your collection is to buy the packs of cards - not individual cards. And, buy from a reputable Pokemon dealer.