Instructions
Check the price and the seller's location. Fake Pokemon games are often sold for significantly less than the real versions. If you are buying from an auction site, be wary of sellers in Asian countries.
Check the packaging. Counterfeit cartridges often show differences in the packaging from the original versions. Labels may be off center or show a different image, the print quality may be lower or the paper used may be different.
Look at the plastic used in the cartridge housing. Games that came with translucent plastic might be solid-colored in counterfeits or vice versa.
Check the model numbers. All official Nintendo cartridges have a model number printed on the label and on the back of the cartridge. If the model number of your Pokemon game doesn't match, it's fake.
While it's not always possible, if you can do so open the cartridge and look inside. Official Pokemon cartridges for the Game Boy Advance used flash memory, so there is no battery mounted on the circuit board. With cartridges that have translucent plastic, sometimes you can see whether it has a battery without opening it. If you find a button battery in the cartridge, it's fake.