Why Chanel behaves differently to every other house
A quilted bag is a depth problem. The quilting has to stand up, hold its diamond spacing and spring back when pressed. Flat quilting is cheaper because it uses less leather and less time, and it reads as flat exactly once you know what to look for - in the shadow line between two diamonds, under side light.
The chain is the second honest test. Chain weight comes from link thickness, and link thickness is visible in a macro frame against a surface for scale. It is also the part of a flap bag that fails first when it is under-made, so the question is both about quality and about longevity.
The third is the flap. A flap that sits level when closed, without a lift at one corner, tells you the panel was cut and assembled with care. It costs nothing extra to do properly and is one of the most common things done improperly.
Written for Chanel specifically. The same discipline applied to a different house produces a different list - see the brand index.