Redox definitions

In the Brønsted-Lowry model for the behavior of acids and bases the primary characteristic of reactions is proton transfer. There is another category of reactions which we introduced early in the year that is also characterized by a transfer of something. Redox (or reduction-oxidation) reactions involve the transfer of electrons. It is now time to return to this area of chemistry for a closer look. In particular we are interested in certain kinds of redox reactions in which the electrons can be made to do work during their transfer or in which forcing a transfer of electrons can make a chemical change happen.

First, a short review of the basic concepts.

The substance which loses electrons is said to be oxidized. The substance which gains those electrons is said to be reduced.

How do you know when a reaction is redox?