Natives don't have to memorize the complex rules and endings for different declensions. We use them intuitively. If we want to know which case we have used, we, effectively, replace the noun by "кто" or "что". "The questions" is a common way to do this substitution. This way we just need to remember the case endings for these two pronouns. The method doesn't work in the opposite direction and it is useless for foreigners.