Apart from going to school for several years to study the science of weather patterns and meteorology, there are a few simpler tricks to tell if it is about to rain. The first trick does not always work but it is the simplest. Is it cloudy and are the clouds dark or grey in color? If so, then it may be about to rain. It can also rain when it is clear and sunny as well (known as summer showers) so this trick is not always true but it is a pretty good indicator.
Another way you can tell if it is about to rain is by smell. The air has a specific smell when it is about to rain, and the air feels much heavier and more humid. Look for these little signs to be able to tell if it is about to rain.