If you like the piano so far, the next stop is to look inside. Ask the owner to remove anything that’s on the piano. Then open the top by doing the following:
Grand piano only:
If the front part of the lid is closed, fold it back onto the main lid. Caution: Before opening the lid, be sure the lid hinges are attached on the left and the hinge pins are in place, or the lid may fall. Very carefully open the lid, then prop the lid open with the lid support. Try removing the music desk by sliding it toward to you. Some music desks are held in place by a screw or hinge pins. If a music desk is too tight and difficult to slide, do not remove it.
All upright pianos:
Lift up the top. On all upright pianos, you should be able to lift the top panel. However, there are differences between piano designs on how to do this. On some brands like Baldwin Hamilton, the top and front panels are attached together. For these pianos, you have to grab the panel above the keyboard cover and after you lift it, you will see a support piece on the left side inside the front panel. Move the support peace and secure it on the side of the piano or have somebody hold it for you. Everett studio pianos have two machine screws on the back of the piano instead of moveable hinges and it is not very easy to take them out without a special tool. On most upright pianos, the top panel is separate from the front panel. If the piano you're looking at is like this, then try to remove the front panel. You should be able to see how the front panel is attached (eg some are held together with screws while others sit on hinges) and based on this, can figure out how to remove it. Next, remove the lower panel. The lower panel is usually held in place by a leaf spring or two. Simply press the spring up toward the keybed and pull the panel out. Sometimes a rotating wooden latch is used instead. When replacing the lower panel, be sure the bottom edge of the panel is lined up correctly with the bottom rail on which it sits. Now that you've opened the piano, use your lamp to look inside. You'll need to check out multiple items, as detailed here
or by clicking on the Step 3 link in the navigation menu above.