First, look for the know oxidation numbers.
In this case, you should know that SO4 comes from H2SO4. When H2SO4 loses 2 H+, SO4 stays with charge 2-
Given that CuSO4 is neutral, the oxidation number of Cu is 2+: (2+) + (2-) = 0.
Conclusion: the oxidation number of Cu is 2+
