Digest (dissolve) a small portion of the brass in nitric acid. Then test the sample for copper content using an atomic emission or atomic absorption spectrometer.

I've searched for couple of weeks but I couldn't find any thing

the only way is using atomic absorption spectroscopy

Atomic absorption is more sensitive to atomic emission when the excitation potential is greater than 3.5eV.

Atomic absorption spectrophotometry is a method in analytical chemistry.

Atomic absorption spectrometry is more sensitive than atomic emission spectrometry.