There are several observations which can be made which gives hints as to whether a chemical reaction has occurred.  Those observable changes  are:

1.  When liquid or solid substances are combined and the evolution of gas occurs.  eg: evolution of gas from an antacid in water or hydrogen peroxide in an open wound.

2.  An insoluble precipitate is produced when chemicals are combined in aqueous solutions. eg: the formation of an insoluble scum in a tub after adding soap or bath salts.

3.  The evolution of heat when chemicals are combined.  eg: iron powder in a "pocket warmer" heating when exposed to air.

4.  The production of energy in the form of sound or light.  eg: an explosion.

5.  A change in color which is not just caused by the combination of other colors.  Combining red and yellow pigments making green pigment is not a reaction, however cooking pink, raw meat to a grey or brown color is.

6.  A change in the magnetic or electric properties of the origianlreagents.  eg: heating sulfur in iron cause the iron to lose its magnetic properties