Question

How can you tell if a reaction is an oxidation-reduction reaction?

Answer 1

REDOX is the process whereby electrons are transfered from one specie to another, via oxidation-reduction.

In a chemical reaction, when one specie causes another to gain electrons, it is called a reductant (it gives electrons). This is an oxidation reaction, because it loses electrons of its own. While the other specie goes through reduction from gaining electrons.