Because many, many, many, MANY scientists have done the reactions before and the key part of scientific research is documenting everything you do. Therefore, all they have to do is look it up in a book (if it's not something that they've already memorized) and they have the proper catalyst to use?
Now how did that particular catalyst get discovered in the first place? Experimenting. For millenia scientists have tried using different chemicals to speed up the rates of reactions. Early on, it wasn't exactly safe or wise, but they would just take a random chemical and throw it into a mixture to see if it had any effect. Nowadays, the structure of the chemicals in use, the reaction pathways, and various other characteristics about the reaction itself are used to determine beforehand if a catalyst exists for that reaction and if it would work well.