Bah - so I just got burnt accidentally buying a pirate copy of Metroid Fusion. I did however learn a bunch of good info in the process - so I thought I would pass it along.
1) Real GBA games have ©200X Nintendo on the board - right above the pins. You don't have to take the cart apart to see this, just look down the pin end and it will be there.
2) The game should have a number indented into the label, usually on the right hand side. Sometimes on a used game though this is really hard to see - but if you tilt it right, it should be there.
3) Nintendo has a pretty good site - ap.nintendo.com - that shows the most common pirated games. You can check the label on your cart versus the known good and known pirate ones.
It used to be really easy to tell the fake ones - as they had mispellings on the cart. The new ones are much better, so you really have to be careful when buying used or on ebay. I never would have known the difference. I only started getting suspicious when the cart was freezing up on me.