Usually there is a plating/painting layer on the surface of metal eyewear, there are following simple aspects to judge the plating or painting quality of metal eyewear.
Appearance
Observing metal eyewear plating layer, no pitting and uneven place, if there are pits or uneven place, the quality of metal eyewear is relatively poor.
It also can try to get metal eyewear cross on white A4 paper, see if there are any fading.
Touching
Metal eyewear under the rub with fingers, or gently claw with nails, see if there are any fade or the fingers tinted with painting colors. The quality of metal eyewear better, the fade or tinted won’t occurred.