To tell a Cu(NO3)2 solution from a Ni(NO3)2 solution, you can add KI to the solution. If the solution contains Cu(NO3)2, the brown precipitate will be formed, but if the solution contains Ni(NO3)2, no precipitate will be formed.
This is the best way to distinguish between these two solutions, since adding KOH or Na2C2O4 will result in blueish precipitates, being harder to identify if it is Cu(NO3)2 or Ni(NO3)2 and with Na2SO4 no reaction will happen.
It means that the answer is: By adding KI and seeing if a brown precipitate is formed.
