Spring hinges are generally a good indicator of quality. They are more expensive to install and to integrate into the final product, and so tend to be included in the better quality frames. When grasping the side arms of the frame, you should be able to lever the sides away from the orthogonal by about 20 degrees without applying any significant force. You can easily see if the hinges are built to accommodate this. If they are, then they are spring driven and supported.
Pads that are mounted on metal need to be silicone rather than rigid plastic for a more comfortable fit. The silicone pads normally feel slightly softer or giving to the touch – so compare side by side with rigid plastic until you get a feel for it!
Always look for a “CE” mark on the frame. Any glasses manufactured or sold into Europe normally require the labelling to be printed on thee frame, and again implies that the manufacturer has a certified level of acceptable quality. Lack of it is more of a warning sign rather than the presence of it is a guarantee of quality however.
If you are considering a plastic frame, ask whether it is made from plastic or acetate. The latter is much higher quality for material construction and will wear much better over time.
Frames should be made from a nickel free material – or better still Titanium – as it is quite common for the skin to have reactions to nickel based or infused material over time.
Do the obvious – and handle the frames well prior to purchase if possible. Most people can discern the quality simply by feeling the balance and movement of the frame on their face.
Be cautious if you are considering “gold” frames – as the plating used on modern frames tends to be very thin relative to your old pair ! The coating can easily be rubbed off if it is in contact with your skin, in less than a year.