As a side note copper alloys have better corrosion protection than
zinc, cad etc... and corrosion is the #1 failure of brake systems. The big NO NO is pure copper tubing like refrigeration tube or the hardware store stuff and the visual difference makes it tough to tell apart. Second most common failure is cracking / breaking of tube.
I would think a copper alloy coated tube would be great and as mentioned a magnet would identify the presence of steel under coating. In this world of extreme liabilities and SUE happy world I too would back off from what looked like pure copper tubing. The alloy products designed for brake systems are not magnetic and would give me no way to tell from refrigeration tubing so I would also refuse to work on the system just because of liability. Maybe not fair but you must protect yourself anymore.