Which method of testing substances is a sure way to identify a chemical reaction?

Hi

Although there are around four important indications which can be used to identify a chemical reaction such as production of a gas or solid substance as a precipitate, changes in color of reactants, a transfer of energy happened in the reaction.

However if we talk about most observable way to identify a chemical reaction then the color changed produced in the material are most sure way to identify the reaction. A very common example of the change in color as a result of chemical reaction is, when litmus paper is placed in acidic or basic conditions.  In acidic conditions, Blue litmus paper changes into red while in basic conditions, red litmus paper is changed into blue.

Hope it helps!