The best test is your ears and inspection of wire strength as strain relief is usually done by nylon or some plastic fibers that coat the wire need to stretch less than copper to protect the copper from becoming brittle.
Strain relief at both ends is critical for reliability. Compare Apple earbuds with cheap earbuds. "Smokin Buds" brand tend to be good value but not necessary highest quality.
Quality tends on diaphram size, material neodinium magnet size, gap control of moving coil, 60 Ohm range is nominal good value for low but adequate power.
I recall 55 yrs ago as a kid a crystal earbud tightly sealed near eardrum had better bass response with a large crystal wafer than modern earbuds.
You can create your own test patterns for frequency response with Audacity and listen for poor resonant vs flat response and use mixed sweeping sinusoids and listen for aliasing harmonics +/- differences of the order of 10% vs earphones <1% due to the nonlinear response of small rare earth magnets with cone deflection.
One could get more technical and model the ear canal and couple the earbud o a high quality magnetic microphone to capture the frequency response and harmonic distortion in Audacity.
But having done many component qualifications, I could give more detail to define the tests I would do to verify cord reliability with yank tests to failure and sound quality tests, but your eyes and ears ought to be good enough to compare each brand samples with cost.
Gold plating is always used for all relay contacts < 2A to avoid the oxidation that occurs . But nickel plating is pretty common (shiny metal) and often adequate and sometimes not. The gold would likely be flash plating 1~2u" may be a nice to have but wire strength, vs flexibility and sound quality with cost I think are the 3 top variables.
p.s. I just remembered that silicone size adapters were a great feature of Smokin Buds that help with air sealing and ear retention with 2 or 3 sizes included for different ear canal sizes. The hard plastic are easier to keep clean but easier to fall out and harder to fit a wide range of ear sizes. The best ear buds for comfort are almost invisible and create no high pressure feeling yet stick to the ear canal with the rubber like silicone adapter and dont have to be expensive but well designed.
A collar clip is also handy to drop ear buds momentarily without actually rolling them up. Also package container for storage and reuse is an important factor. THe round windup spools are better for protecting the fragile stranded AWG 48~50 wires.