The best way to find out would be to plug in an external display. Take the bottom off, unplug the DB-15 connector, and then connect either a multisync vintage Apple display, or a VGA adapter and VGA display. If you get an image on the external display (the internal display will remain dark and the power light amber because the display's not getting a signal) then its a display issue. If you still don't get an image on the external display I'd be looking at your RAM/VRAM, or perhaps the slight possibility of a busted GPU.