How to find out the variable names for debconf-set-selections?

Let's say I want to install mysql from a script without being asked any configuration questions like what root password I want to set by apt. I would then preset the debconf variables: echo mysql-server-5.5 mysql-server/root_password password xyzzy | debconf-set-selections echo mysql-server-5.5 mysql-server/root_password_again password xyzzy | debconf-set-selections I got this from a tutorial. What is unclear to me: How did the guy find out the variable names? How did he knew that he had to set mysql-server-5.5 mysql-server/root_password password and mysql-server-5.5 mysql-server/root_password_again respectively? I know I could extract the .deb package by issuing dpkg-deb -R package.deb EXTRACTDIR/ - but I don't see where those variables are stored. How would I find out the debconf variables for any other package?