Taking the front board off an upright piano will give you an instant assessment of what you are buying, how long it will last and how much you should pay for the piano. The front board on an average upright is usually secured by two small catches located inside the piano on either side. Once these are released the board should lift off revealing the action and strings.
Grand pianos also have dampers and strings etc. Viewing them is much easier and can be seen by looking down into the action from the top. The same issues apply to grand pianos as were applicable to uprights: Frames can crack, soundboards can split. Examine carefully.
Type of Action
What sort of action is it? If you can see the hammers that hit the strings clearly, with no metal struts obscuring them, then this is an underdamped action and you can rely on this type of mechanism to be responsive.
If, on the other hand almost every hammer is partially hidden by a metal strut connected to a wooden rail running over the action, then this is an overdamped mechanism. Generally speaking an overdamped action tends to be cumbersome and unreliable when damping. It works on gravity, rather than springs, and wears out quickly.
Play a few notes and you will quickly be able to determine how worn the dampers are. If the piano sounds as though the sustaining pedal is being depressed then the dampers are worn and the piano should be avoided. This test applies to underdamped pianos as well. On underdamped pianos poor damping is likely to be due to weak springs rather than worn felt. In any event you can expect a large bill for repairs should you purchase such an instrument.
Take a look at the strings. If they are rusty then the chances are the tuner is going to have a hard job putting the piano into concert pitch without breaking strings. This can be costly. If the strings are rusty it is more than likely that the piano has stood somewhere damp at some stage and other items, such as felt and bushings will be affected.
A piano that has stood in the damp for a prolonged period will give considerable trouble when the woodwork and felts begin to dry out. When this happens an expensive rebuild is often the only answer. Avoid purchasing such an instrument.
On playing a chord or scale on the piano make a note of how out of tune the piano is. If it sounds discordant and unpleasant it is likely that, either the piano hasn't been tuned for a considerable number of years, or the tuning pins are loose. Tuning can be expensive, especially when the tuner has to call several times to get the instrument to stay in tune. Unless a piano has been regularly maintained you are going to incur further expense getting the action and tuning sorted out. Consequently it is a good idea to try and obtain something of the maintenance history from the seller.
Loose pins are also expensive to replace and could indicate that the block the tuning pins are secured into has cracked. This type of damage is cost prohibitive on an ordinary piano.
Older pianos are often 'straight strung', that is, the strings do not cross one another at any point in the piano. The strings run from the top to the bottom of the frame in parallel straight lines. Straight strung pianos are acceptable provided they are in good condition. However the sound is less resonant than an overstrung instrument.
An overstrung piano has some of the strings crossing over the others at a sharp angle, thus giving a longer length and better sound and is therefore more expensive. It is well worth paying the extra money.
Lastly, two important parts of the inside of a piano should be scrutinized. First the frame. What sort of frame is it? Does the metal 'harp' like shape of the frame come right to the top of the piano, or does it finish just under the strings, or even halfway down the piano?
A full frame piano is a strong, stable instrument and therefore should be bought in preference to a three quarter frame or half frame piano. A three quarter piano can be suitable if that is all that is on offer. Avoid half frame pianos and do not buy a piano with no metal in at all as it will not last, once it is exposed to modern central heating.
You should check the frame to make sure it is not cracked anywhere. If it is, then the piano is ruined and is only fit to be scrapped.
Secondly, the sound board. This is the wooden board that you can see behind the strings. If this is split badly then the sound will be impaired.