Not a technician, but shopped used pianos for a while and the salesperson offered a few pointers.

• Examine the soundboard under the piano. Yea, get under there. Look for any damage, insects, warping, etc.
• Inspect the case that holds the board
• Inspect the metal plate or “harp”
• Inspect the pins and pinboard carefully, looking for loose ones or repairs
• check for rust anywhere
• pull out the keyboard and inspect the action
• check the felt on the hammers to see how worn they are
• inspect that the foot pedals work
• check that all the keys are level and should respond well
• look around where it’s placed, near windows, etc. since changes in humidity and temperature can adversely affect it over time
• of course play on it and listen carefully

This one looks like it was painted over perhaps? Not a great sign. Best of luck to you!