When we are at the level of purity 99.95+%, when alloy contain say 0,01-0,02% of contaminant, aside of copper, i don´t think you could reliably tell only by looking at the solution. For 99,9 % could be clearly possible (to see mainly copper-ammonia). I encountered tin once as the contaminant in the silver bar, if I remember correctly it was somewhere around 0,05%. We refined this anyway in the cell. Small piece of this (not completely) pure silver was dissolved in nitric.I just use some analytical grade nitric acid, and look to see that I have a clear solution.
Even the slightest impurity will affect the colour.
I do have a spectrum analyser, but have never really needed to use it.
And i can say that solution was crystal clear, without milkiness.