What to check when buying a used device I'm thinking about buying a used Android device and I want to know what should I check on the device before I buy it?

Is there an app or something that does some sort of testing on the phone? 

Are there any problematic parts that I should check?
Don't forget to check for water damage.


  Every phone that is sold by a reputable retailer
  (T-Mobile, Cingular, Verizon, Sprint, etc.) should contain a tiny
  indicator that clearly shows if water has attacked your phone. This
  little item is typically called the liquid damage indicator, or LDI
  for short. In almost every case, a red or pink indicator means that
  there is water damage. A white indicator means that there is no water
  damage.


Here is the detail.