How to identify which species will undergo Oxidation and which will go Reduction I have been given $\ce{B2}$, $\ce{N2}$, $\ce{(O2)-}$  and $\ce{O2}$.

I have to short out  which will undergo oxidation, reduction or both.

How can I tell it when reactants are not given?
You have to look at the possible oxidation states of the atoms involved.
For instance, if you were given the molecule $\ce{F2}$, you should consider that fluorine has two possible oxidation states: 0 (in the $\ce{F2}$ molecule) and -1.
Hence, $\ce{F2}$ can only undergo reduction.
For the other molecules, you should use the same strategy: try yourself!