I guess the easiest way to tell is to have an original screen to compare it to. When I replaced the screen on my iPad 4 I noticed differences when I compared them during the repair. With the screen off, if you tilted it in direct light, you could see a grid on the entire screen that looked like graph paper. On the original screen, the grid was very tight with a lot of squares per inch. The aftermarket screens grid was more spread out with larger squares so less squares per inch. The ribbon cables lacked the quality of the oem screen also.