You learn chemistry till you know.
Seriously, there is no simple, one size fits all kind of answer. Broadly speaking you have to learn about hundreds of possible reactions to see the patterns in the data. That being said, at HS level you should be not asked about reactions you have not yet seen or had no occasion to learn, so most likely just a several basic types will do.
Besides, "chemical equations" don't have "reactions". My bet is what you are really asking is "how do I know if the given combination of substances will react".