Well, you can start with the shutter-count- it will give you an idea of how much the camera has been used. --->
Check Shutter Count & Camera Manufacture Date Online - EXIF Reader
Also, check the serial number in the database; see if he added it here and if he left a comment on it. You'll also get the date of manufacture/serial number so you can look it up in the database and see if the camera might have come from a bad batch.
Check that lens release button feels fine and isn't loose (there's been reports of some falling off).
For sensor stains, you'll want to set it to a high aperture (16+ should work) and point the camera at something like the sky, take a photo, and inspect it on your computer. Sensor stains look a little like trails of dots, but if you see one dot or speck here or there, it will most likely just be dust.
Not sure how to address mirror flop or check for that....
Also, as with any camera, you'd want to check that all accessories are there, that all buttons work, all functions work, AF works, no signs that the camera was abused/dropped, nothing loose and those sorts of things.