The obvious springs to mind, but supposedly it is in a sealed Samsung box, all accessories wrapped properly, the phone is in a cellophane wrapper and the camera and flash/heart rate monitor have screen protectors (the latter being minute and a bitch to get off) then you switch it on.
On switch on, is the battery fairly well charged ? Then from the dialler dial *#0*# to get a diagnostic menu and check everything there.
After your satisfied test the camera, put the phone in standby mode and shake it next to your ear. You should hear a gentle rattle - this is perfectly normal. It part of the image stabilisation of the S6 you can hear. Now immediately open the camera from the lock screen and take a picture (of anything - just be as quick as you can). It should be crisp and in focus.
Then we're back to the obvious again, set your phone up, test as you go, notice how the battery does.
A useful thread, I hope others can add to.