1. With the top lid open, look down inside the piano.
2. Whilst looking down inside the piano and looking at the back section, you should see approximately 250 tuning pins exiting the tuning plank.
3. Have a look around the tuning pins, if you can see or feel a metal structure running the length of the piano (Usually a gold colour) around the tuning pins, the piano has an iron frame.
4. If there is no metal structure around the tuning pins, the piano is most likely a wooden framed piano and may not be tunable.
Below is a photo of what the inside of a 'Zimmerman' brand upright piano looks like with the top panel removed. Note, the large gold coloured iron frame surrounding the tuning pins of the piano...