I often get these questions from agile people (not saying you're or aren't) and my answer is always: the same code reviews that ensure there aren't code smells.

I often get these questions from agile people (not saying you're or aren't) and my answer is always: the same code reviews that ensure there aren't code smells. Lack of documentation is a code smell to me. To be clear, if docs are just translating to English what the code is already expressing then they are bad. They should be high level, explaining odd cases, etc. If they go out of sync, you fix them because your future ability to understand the code depends on it. I've had PR's rejected because they only added comments saying that "they will get out of sync". I've had documentation projects canceled because "they will get out of sync". It makes me really sad when a new hire struggles to understand basic things in our code base. </rant>