How to tell which substance oxidizes or reduces?

Maybe not so simply put, but chemistry is understanding which atom or molecule wants those electrons the most. Fe would rather be as Fe3+ than Fe2+. H+ would rather exist in H2O. Cr6+ would much rather exist as Cr3+. To finish your example, Cl- is happy as is and it would take alot of energy to force it back to Cl2. (ClO- is not very stable).