Looking at a net ionic equation how can i tell with reactant ions were oxidized

You can just look at its oxidation number. If the oxidation number of an atom or ion increases, then it undergoes oxidation.

The oxidation number is same like the charge on an ion.

ex. iodide ion has a charge of - 1, so it's oxidation number is also - 1.

When iodide ion becomes iodine,

its oxidation number (charge) increase from - 1 to 0, so it is oxidized.