At the same as when buying any equipment:
-the presence of chips\\scratches, particularly denting. If the laptop has been dropped I wouldn't buy it.
-the presence of dead pixels or other screen defects
-the configuration stated by the seller
-battery life (or Everest, or just switch on, measure how much is left %, spend all other assessment manipulation, it will take at least ten to fifteen minutes, and see how the battery). For netbook battery life one of the most important parameters.
-check the operation of all buttons
-check the operation of the periphery where you will be able to: webcam, card reader, usb ports and headphone Jack
-in case of any defects feel free to haggle if the seller quickly and easily make concessions: cheerleaders and look carefully
-listen to the inner voice. He is not lying :-)