1) Remaining warranty, if any
2) Check screen quality (destruction due to pressure, dead pixels, discoloration)
3) Original software discs that came with the computer
4) Listen close to the fan (under asdw keys) and hard drive and look for sound abnormalities.
5) check battery quality, new macbooks last for about 4-5 hours on a full charge. my 1.5 y.o. macbook lasts for 3:30-4:00 hrs now...
6) cd-rom, make sure it loads a disc, check disc utility to see if it is recognized
7) iSight, make sure it works with photobooth
8) mic, click sound in preferences panel and click the input tab to observe the mic input fluctuations
imo, i'd buy refurb from apple or at least make sure the person you are buying from has some remaining warranty on his mac.