Only after purchase I know that my IPhone 4S would have 14 day exchange guarantee, if I find any problems. I didn't pay too much attention and the staff wasn't very nice and helpful when she knew that I was a newbie. Now, I still have the chance. I would like to know what I have to check very carefully about the machine. One of my puzzlement is about how to get the full screen or if there are any Apps to assist me checking dead pixels, yellow lines / splotches etc. Thank you very much.