Another easy way to tell is just look at the label. I've yet to see a bootleg GBA cart that didn't have a label that was applied poorly and crooked, unlike machine applied labels to genuine carts. Also, I've never seen a bootleg that had that pressed code on the front label.

Or just insert it into your system. Due to how the cartridge casings are manufactured by bootleggers, they don't insert as easily as a factory cartridge since the casings are slightly out of spec due to how they derive their moldings and produce them.