Of course you want to find out the original year of construction. In some cases this works quite accurately because, for example, a date is hidden somewhere inside the body or somewhere on the neck. But in practice you cannot always dismantle everything before you decide to purchase. Serial numbers are a good indication.
With the older Fender guitars, for example, that number is on the metal neck plate, which can easily be replaced in the meantime. Later copies have a serial number on the head of the neck as standard, but a neck can also be exchanged with a Fender.
Also look as much as possible at the version and the other components to check whether everything corresponds to the same year. For example, potentiometers are sometimes provided with a number where the last 2 digits indicate a week number, and the 2 digits before that indicate the year. For example, 6648 could then indicate week 48 of 1966. The potentiometers will then have to be about the year of the guitar, but may also have been made a year before the guitar was produced. It is only an indication and for this you also have to start "screwing". You can also sometimes find indications on the elements that refer to a year. This can vary greatly per brand. See also the various links below. Some manufacturers respond nicely to e-mail in which you request the data on the basis of the serial number.
The above more or less also applies to amplifiers. There, a code on a transformer can also be an extra indication to find out the year. The production date of the tubes may still be known. In many cases, these will have been replaced in the meantime, but you will get an impression of the age of the current set. Pipes deteriorate in use over the years, however, unused pipes will not really age. Frequently heard cries are "NOS" (new old stock) or unused tubes from an old stock. "NIB" (new in box) refers to an original old production tube that is subsequently packed in a (usually white / blank) box. New tubess are still being made in some factories, whereby those from China are valued less.
What I regularly benefited from, was first to search through Google for an identical object of the same year. I got photos and details that served as a good comparison material. You then know what to look for and immediately get an impression of whether the asking price is realistic. I even came across the same instrument in an ad from a previous seller who had once offered it. Also remember to search by brand / type but also with the text "catalog". Several old brochures with the original specifications and images can be found on the internet.
With the "better trader" you can actually expect that everything has been checked beforehand. For completeness, pictures have already been taken of the various details so that the instrument does not have to be disassembled again. People will also have the knowledge to find out the possible counterfeits in time. Did you know that, for example, the logos and even numbered neck plates (serial number entirely at your choice, see result of sample on the right) can be ordered via eBay?
With regard to codes and serial numbers, there is plenty of information available via the internet. It would be superfluous to place such lists here, so I limit myself here to mention a few links to make it a bit easier. . .
