Many fake cards are made of thinner plastic and allow light to shine through (you can see the Poke Ball on the other side of the card if you hold it up to a well-lit ceiling). The writing is kinda off and letters look much thicker than normal. Also, the packaging on fake boosters is loose, unlike with real boosters which wrap tight around the cards inside. Needless to say, stay away from booster packs that have pictures of Pokemon with incorrect colours or crude artwork.
And as blunt as this is, please don't buy Pokemon cards from your local convenience store or from supermarkets. It's not always true, but those cards are usually fake.