I dont know if theres a purity test for copper that would be easy for DIY garage builders. Even if there is some admixture from a cheap copper recycler, just check the resistance in ohms with a multimeter.
The reading wouldnt have to be precise. As long as it's very high conductivity and very low resistance, its miles ahead of using pure nickel. Check a longish section of your copper, and then check the same length on a section of copper wire that you are certain is pure copper.
The IACS conductivity of nickel is around 24/100, and pure copper is 100/100, four times better.
If you got ahold of some dirty recycled copper that was only 98% copper, you are still doing great. Steel is roughly 5/100, which is horrible.