I used to sell pianos and inspect trade-ins. What quiltguy said.
In addition to simple wear and tear, the piano may have obvious damage that causes harmonic rattles or buzzes. Here’s what I’d advise you look for.
First, I agree with bringing someone along who knows how to play the piano. This person would probably be able to feel that it “plays funny” or that something doesn’t feel right about the keys. You might also ask the owners how long it’s been since the thing was looked at by a tuner and/or repairman.
Sound every key to see if there are broken keys, missing hammers (the key plunks down but there’s no tone) or missing dampers (the note rings out but it doesn’t stop. Remember, the upper range of the piano likely doesn’t have dampers by design) or double-strikes (you push one key but two notes ring). These problems are often relatively easy and inexpensive to fix.
To amplify what venterap is saying about the piano’s frame, here is some good piano vocabulary. In an upright, the piano strings are wrapped around pins and held in tension across a bridge made of hardwood that directly contacts the sounding board. You know how a violin has a wooden bridge that conducts sound between the strings and the resonating hollow body of the violin? Same thing. The bridge conducts the sound to the sounding board, amplifying the volume of the strings. The bottom end of each string is affixed to a pin in the frame, which is the big harp-shaped metal construct inside the piano.
If you can see the back of an upright piano, you might wish to examine the sounding board—but in all honesty it’d probably be easier to just play the darn thing and see if it has a good sound. Cracks in a sounding board, even big cracks, don’t appreciably change the overall sound-producing power of the piano. However, major damage to the sounding board (not unheard of) can change the tune of the strings.
When we say the strings put pressure on the bridge, we ain’t kidding: sometimes as much as twenty tons of pressure for a fully, properly tuned piano. This puts a great deal of strain on both the metal frame and the wooden cabinet, so while you’re nosing around the back of the piano, make sure everything is straight and undamaged. The back of the piano is reinforced wooden posts to keep the piano itself from twisting out of shape under the tension of the strings.
The sounding board has to have a nice gentle crown (which you probably can’t actually see) in order to produce volume. It’s similar to the shape of a speaker cone. If the sounding board is bad, you’re talking big bucks to fix it. More here.
Make sure that the sustain pedal, at the very least, does what it’s supposed to do: that is, when you hold down the rightmost foot pedal, all of the notes should continue to ring even after you release the key. When you then release the footpedal, all the notes should then be damped at the same time. Depending on how the piano is put together—and there’s a lot of variation in uprights—you may have a practice pedal, a soft pedal, or a sostenuto pedal (or two of the above).
Open the top lid (assuming it’s an upright, cabinet, studio, or spinet piano) and remove the front faceplate. Usually there’s a swinging arm on the left and right that you move up to disengage from a peg on the face plate; then the face panel lifts up and out.
Check to see that there are no obviously missing strings. The lower notes are 1 string per key; the low-midrange are 2 strings per; the rest should be 3 strings per. Since the low and low-midrange notes are string coils rather than straight strings, look for rust. Sometimes very old and/or rusty string coils will snap when they are tuned.
Look for any obvious cracks or major damage to the frame (the metal part of the piano inside).
As a side note, the hammers are generally made of wool. Over time the hammers dry out and harden and this causes the piano’s tone to become sharper and a bit, well, twangier. This is pretty much a consequence of buying an old piano. My 1913 Estey had the hammers completely replaced before I bought it, so it’s got a nice mellow sound now.
Depending on whether the piano is an instrument or furniture, do this first or last, and check the cabinet for marks. Be sure to fold out the keyboard cover (most pianos have one that retracts or folds and hides) and check the top of that too. Lots of pianos I have seen, especially old ones, have distinctive marks where a player left a burning cigarette on the edge of the piano. Drink rings are also favorite. Also, if you like your piano to show its teeth, make sure the ivories are all attached. Many old pianos have missing ivories, though this doesn’t affect performance.
When you move the upright in, give it a few days to settle into the room’s temperature and humidity before you try to tune it.
FISH