It depends, but usually a process that is spontaneous is exothermic. There are plenty of counterexamples to what I just said, but it usually holds true for VCE.

This means that reactions that happen naturally (i.e.: spontaneous) are often exothermic.

The intuitive reason behind this is that in exothermic reactions (where energy is released to the surroundings), it wants to escape from being bound into a tiny bundle of space, or a much greater space. The energy wants to be free and spread out. That should make some kind of sense I hope, it works for me! Hence, exothermic is often spontaneous.

Usually you will be told -- you shouldn't be expected to know which reactions are which. There is no "theory" behind it. Chemists still refer to tables of data measured in lab experiments to figure this out.