From 1984 Chanel placed an adhesive serial sticker inside the bag, protected by a clear film, with a matching card in the box. The leading digits are a series number, and each series corresponds to a production window — that is what lets you test a bag's claimed age against its hardware and lining.
Seven-digit serials belong to the early series and eight-digit serials to series three onward. The relationship between series and year is well documented and roughly linear, so a bag whose serial places it in 1994 but whose interior is a modern microfibre in a colour Chanel did not use then has told you something.
In 2021 the sticker was replaced by an NFC chip sewn into the lining, and the authenticity card went with it. Recent production therefore has no visible serial at all, which is correct.
Two mechanical checks on the sticker itself: the film should be intact and bonded, with the digits sharp and evenly spaced under it, and the sticker should sit on lining that has aged with the rest of the bag. Counterfeiters reuse genuine serials, so a number that checks out is not proof — but the physical sticker is hard to reproduce well.