Software ones you cannot look out for... in that case you just download the latest beta OS from this forum, and if you encounter any software problems after that (bugs), you report them to Sequential.
Hardware wise... well... if you buy a 16 voice REV2, I'd double check that it is in fact a 16 voice, and not an 8 voice... hold down 9 keys (pressing them one by one) and notice if the lowest you pressed goes away on the 9th keypress... if it does, you're dealing with an 8 voice (unless stack or split mode is on of course).
Other than that I'd simply check that all keys are functioning, that buttons are working and knobs and wheels too.
Eventually check the inputs and outputs on the back of the synth to ensure that they work (for that you may need to bring along a foot switch, foot pedal and a pair of headphones etc.)
If you have the option to see it connected to a computer via USB or MIDI DIN you could check this too.
Other than that, I do not see any other things to check for really... in most cases, what you want to worry most about are the keyboard and the wheels, buttons and knobs... these are the ones that is usually worn down on synths, plus the occational audio outputs that may be defect (scratches and noises when you wiggle the plugs).