Determining glasses frame quality is quite difficult, even for an eyewear industry professional. The most reliable determinant of the quality of a pair of glasses frames is the feel of them in your hand. Do they rattle? Do they feel stiff and solid? When you exercise the hinges do they move smoothly and with the same friction throughout the bend? A better quality glasses frame has a higher level of machined perfection, so that the better quality parts fit together well. Just because a designer glasses frame has a brand affiliation, it does not mean that the glasses frame is of high quality.
Nowadays, about 90% of all eyeglasses are made in China. All of the major Italian glasses frame companies have moved the majority of their production to China. Many of the eyeglasses that are made in China are then shipped to Europe where a minimal amount of assembly or finishing work is done so that the glasses frames can receive the Made in Europe designation.
There are some designers, like Alain Mikli, Lafont, and Silhouette and a few others that make their glasses frames completely in Europe. Brands like Armani, Gucci, and other big fashion houses are mostly made in China, and then finished in Europe. This is not to say that China eyewear is of inferior quality. China has been making eyewear for many years and the quality is quite good.
Glasses frames are a composite of many different parts. Eyewear manufacturers source parts from all around the world, and can assemble glasses frames in different places as well. A plastic glasses frame is composed of the plastic frame front, plastic temples (with wire running through part of the shaft), and hinges. Metal glasses are composed of the metal frame front, metal in the temples, plastic jackets to cover the temples, hinges, frame front, nosepads, and nosepad arms.
Using German hinges and Italian Mazzucchelli zyl (the best) results in an excellent quality European glasses frame. This same frame model could use Mazzucchelli zyl from China (same company, different factory), and Chinese hinges, and be assembled in a Chinese factory. The Chinese frame would be very difficult to differentiate from the original, except that it would be designated Made in China.
Now, let's say that this same frame model used Mazzucchelli zyl from China, and Chinese hinges, but was assembled in Italy, it would be called Italian eyewear, Made in Italy. Most European eyewear nowadays (except for a few high end brands) that is made in Europe is made mostly of Chinese components.
Hinges are a small but very important component in the overall quality of your glasses frame. If the hinge is not mounted on the frame properly, it will disengage which leads to a very difficult (or impossible) repair job. Low quality hinges will wear out, the screw will pop out and get lost, and they will wear quickly leading to rattling or sloppy action. Good quality hinges have a smooth and consistent action that does not wear over time, with friction that is consistent throughout the entire range of motion.
We still find that the best quality hinges are made in Germany. You can find these in European and Chinese frame, and in most better quality frames. Unfortunately, it is very difficult to find out from the manufacturers which hinges they use in a particular frame.