How to check out a used piano: 

·    Open the piano where you can view the action

·    Play each note repeatedly and watch the action

o   The hammer should strike without a wobble, and bounce back to be caught by the backcheck

·    Look for things that are obviously broken

·    Look for broken or badly corroded strings

·    Take something like a pitch pipe or other device with a known pitch.  Does the piano match the pitch for the given note?

·    Press the right pedal and watch the dampers lift off the strings, letting them ring on.

·    Release the right pedal and make sure the notes all cut off

·    Listen for strange sounds and notes that are way off

If the piano looks good on the inside and every note plays well you may be ok.  Obviously, this is no kind of guarantee, only a rough idea of the piano's condition.