Best real tests I know are:
- Look inside at the pin area. Do you see pins that go all the way back into the plastic? - Fake
- Look inside at the pin area. Can you see  NINTENDO 2001? - No? Fake.
- Look at the cart plastic. Is the coloring off and not in a yellowing plastic way? - Fake
- Look at the cart plastic. Is the molding wrong on the edges or the font saying Gameboy Advance looks off or says something else - Obvious fake
- Look at the sticker. Is the ESRB, Seal, Nintendo logo, publisher and developer logos look wrong, missing, or otherwise altered - Fake
- Look at the sticker. Is the sticker art of cheaper quality, image stretched out, look like it ran off a desk jet printer? - Fake
- Check the back with the screw. Is the screw philips or some other non-security bit - Fake again.
Those are the benchmarks of a fake or tampered with cart you should avoid.
- Look inside at the pin area. Do you see pins that go all the way back into the plastic? - Fake
- Look inside at the pin area. Can you see  NINTENDO 2001? - No? Fake.
- Look at the cart plastic. Is the coloring off and not in a yellowing plastic way? - Fake
- Look at the cart plastic. Is the molding wrong on the edges or the font saying Gameboy Advance looks off or says something else - Obvious fake
- Look at the sticker. Is the ESRB, Seal, Nintendo logo, publisher and developer logos look wrong, missing, or otherwise altered - Fake
- Look at the sticker. Is the sticker art of cheaper quality, image stretched out, look like it ran off a desk jet printer? - Fake
- Check the back with the screw. Is the screw philips or some other non-security bit - Fake again.
Those are the benchmarks of a fake or tampered with cart you should avoid.