The thing I've noticed most showing zinc in a melt is a 'skin' forming on the surface even after a good fluxing and stirring. The skin interferes with ladle pouring. Since I'm old fashioned, and have not used my 'new' bottom pouring electric pot as much, I don't know what effect the zinc has on them. It sure doesn't take much zinc to make it skin. Another sign you have a problem, is the presence of a floating silver chunk long after everything else has melted. Mystery metal, Uh-oh!