1) check for matching IMEI (dial *#06#) with phone and box.

2) check all contents are available in box

3) check for dead pixels. use samsung function test by typing this code *#0*# into the dialer. go through all the test and make sure everything is working

4) check for any hardware defects by pressing the phone at several spots for squeaking or weird noises.

5) check for yellow tint, open up a white picture and check the screen for yellow-ish