This is only useful once you have bought the game but as they're illegal you can then demand a refund!

Where the circuit board pokes out at the bottom of the cart, you should see Nintendo printed in white on the board.  This means its real.  Fakes don't have this and instead have a small oblong gap.  Prise it gently apart and you will see a microchip soldered onto the board. This is a fake. 

The other guides on here regarding labels are pretty good but I work in a games shop and some fakes are excellent.  The above is the surefire way to spot fakes.

If you dont mind fake games you can get real bargains and most work fine.  But they are illegal and resale value is minimal.

Happy hunting!