Give the item a tap with your fist and listen closely to the sound that is produced. As copper is a softer metal, it should create a mellower and much rounder sound.

https://www.highettmetal.com.au/blog/copper/how-to-tell-the-difference-between-copper-and-brass#:~:text=Carefully%20scrutinize%20the%20colour%20of,be%20deeper%20brown%20after%20corrosion.