Actually, there are several indicators of a chemical reaction.

• We could see that a smell or odor is given off. Think about the burning of waste material.
• We could see that there is an energy change. The energy may be consumed (endothermic reactions require energy supplied generally through heating) or generated (exothermic reactions give off heat). Think about the burning of coal or wood. This is also exhibited through a change in the temperature of the solution.
• We could see the formation of new compounds, which could be solid, liquid or gaseous in nature. Think about precipitation of solids.
• A color change may take place. Think about rusting, how color is seen on fresh metal surfaces.
• A change in composition may take place.
• A change in volume may take place.