Question

What piece of evidence would prove that a reaction is exothermic

Answer 1

Combustion is a chemical reaction. In complete combustion(which is the only type you'll be needing to talk about more than likely) reactions, a compound reacts with an oxidizing element, such as oxygen, and the products are compounds of each element in the fuel with the oxidizing element. For example:

CH4 + (2)O2 → CO2 + (2)H2O + heat

or even simpler

(2)H2 + O2 → (2)H20 + heat

A chemical reaction is just whenever two chemicals are combined and create a new compound. So your paper is combining with oxygen(under the influence of heat) to create byproducts of whatever was in the paper and oxygen...