How to know when do you need a filling and when do you need a root canal?

Root canal vs fillin: A filling is done when bacteria has caused a cavity in a tooth. If the cavity penetrates deep enough into the tooth an infection can occur causing pain and swelling. This is an infection of the root canal system. In the latter case you'll need a root canal. If your dentist tells you that you have a cavity, don't wait, it can and will progress to needing a root canal over time.

Depth of damage: Root canals are fillings. When damage/decay/fracture/bacteria find access into the pulp chamber system inside a tooth, a root canal filling becomes an option. A filling, crown, or partial crown (inlay-onlay) usually works for damaged teeth that do not have pulpal involvement.