When you're shopping in a store that has a ton of loose carts, tilt up and look inside the ones you want to buy. Make sure there is a copyright Nintendo logo. If the game contacts are long and extend all the way up into the cart, it's likely fake.
I've also noticed that a lot of bootleg GBA carts have the wrong ESRB rating printed on their label for some reason. Find a copy of Mortal Kombat Advance with an E rating? Set that motherfucker back down and walk away.
My local record store is one of the few places around that still sells GBA carts. I alert the store clerks if I see any bootlegs. Sometimes they seem kinda irritated, but I'm just trying to help out.
