Given the redox reaction: ni + sn4+ → ni2+ + sn2+
which species has been oxidized?

Remember this---> LEO=  lose electons oxidation... and GER> gain electrons reduction

so if you look for the one being oxidized, it is the one losing electrons. since electrons are negative, when an atom lose them, it becomes less negative, so more positive. 

you can identify the oxidization when the atom becomes more positive.

so in the given reaction: Ni + Sn4+ → Ni2+ + Sn2+

Ni becomes Ni +2   and    Sn+4 becomes Sn+2

as you can see the Ni become Ni+2 so more positive

Ni is being oxidized

If you explain it more through i will answer it : )