Answer:
Initially he should measure that how much time it takes for the liquid to turn clear if X and Y are mixed together. After that he should find out that how much time it takes if he also supplements substance A to X and Y, to find that whether the substance is a catalyst or not.
Step-by-step explanation:
Determining the time consumed for the liquid to turn clear when X and Y were combined together will give a prediction about the rate of reaction and the time of reaction.
The same method should be followed when component A is supplemented to the liquid X and Y mixture. If a fast reaction takes place with the supplementation of A to the reaction mixture then it can be said that A functions as a catalyst for mixing X and Y. At a similar time by comparing the rate of reactions of both the reaction it can also find out whether A is a catalyst or not.