Wear an outfit that you don't mind getting dirty. Bring a flashlight, pen and notepad with you to every piano you inspect. Open the lid and examine the tuning keys.
Press every piano key, beginning at the left end of the keyboard, to make sure each key works. Listen for any buzzing or clicking sounds, and make a note of it in your notepad. Write down the piano brand and serial number, which should be five or six digits long and stamped somewhere on the cast-iron plate inside of the piano.
Use your paint brush to gently brush away dust that has accumulated on the tuning keys and bass bridge. Examine the bass bridge for cracking. If you see cracks in the wood where the bass strings stretch across the bridge, reconsider purchasing the piano. The overloaded tension on the strings causes cracks and makes properly tuning the piano impossible. Consider the brand and overall condition of the piano. An expert piano repair person can examine the piano to see if it warrants the cost of internal reconstruction.
Lie down on your back, beneath the piano and shine your flashlight on the soundboard. Look for any visible cracks in the soundboard and the bass board. Press gently with your thumb against the bass board near each rib. Feel for any spots that may have become unglued. You will hear a buzzing sound when the bass keys are played if spots are unglued.