How does a student know that a chemical reaction has occurred?
How else but by observation.........?
And typically chemical change is accompanied by a colour change, or by the formation of an insoluble precipitate or by the evolution of a gas. Some chemical reactions are more obvious than others.
You won't be asked to interrogate a process to which you haven't been introduced. Most of the time, designation of a reaction as chemical or physical is pretty clear:
In general chemical change is characterized by the formation of new substances and the making and breaking of strong chemical bonds. Physical changes are largely changes of state.