Question

how do you figure out which species is the spectator in redox equations?

Answer 1

First, write out the equation. Then, separate the redox equation into its two half-reactions. There will be an oxidation half-reaction and a reduction half-reaction. If you see that one molecule or element is on both sides and doesn't undergo any reduction or oxidation, and just stays the same charge throughout the entire equation, that's going to be your spectator ion.
Hope this helps!