How do you know if it is an oxidation reduction reaction?

Oxidation numbers are theoretical constructs, and have marginal physical significance. Formally, the oxidation number of an element in a compound is the charge the element would have if all the bonding electrons were broken, with the charge assigned to the most electronegative atom.

For the molecule #H-F#, we break the bond, and the 2 electrons are assigned to the fluorine centre. I reiterate that this is a theoretical exercise:

#H-F rarrH^+ + F^-#

Thus fluorine is conceived to have an oxidation number of #-I#, and hydrogen an oxidation number of #+I#; Roman numerals are used.

So if oxidation numbers change upon reaction, electron transfer and redox has occurred. Typically powerful oxidants such as elemental fluorine, #F_2#, and oxygen #O_2#, after reaction (after oxidizing something) assume oxidation states of #-I# and #-II# in the product compound.

Oxidation of hydrocarbons underpin our civilization. If we look at the combustion of coal, we can write the equation:

#C(s) + O_2(g) rarr CO_2(g) + Delta#

The oxidation number of an element is formally #0#. The oxidation number of #C# in #CO_2# is #+IV# and of oxygen #-II#. Electron transfer has taken place between oxygen and carbon when the reaction occurs, and thus hydrocarbon combustion is formally redox.