There is a way.
Look on a fserve for the rom color.gba
flash it in your card of choice and use the L/R buttons to cycle through colours, that way you can check for dead pixels, to test the other screen, just go into options then select gba playback on the 2nd screen.
Even though gba games do not fill the whole screen, if you are in a room with good light you should be able to notice any dead/stuck pixels around the screens edge.
Good luck!