1. Try all the pedals. Listen for squeaks and noises.
2. Play all notes throughout the keyboard. Make sure they sound and return correctly.
3. Look inside the piano. Look for worn or broken parts. Ask questions about the history of the piano and whether or not is has been maintained regularly.
4. Have the owner/salesman play the piano for you or play the piano yourself to determine whether you like the tone quality and the feel of the pianos action.
5. Be willing to learn and take advice of knowledgeable piano people but inevitably make your own decision about the piano and your purchase.
*These are just some quick checks that can be made by the average piano shopper. It is always advised to have a qualified technician evaluate the piano thoroughly prior to purchase.