Giving apt-get the -u option causes it to print a list of all packages that are to be upgraded and to prompt the user before taking action. The following makes apt-get always do this:
$ cat >> /etc/apt/apt.conf << .
// Always show packages to be upgraded (-u)
APT::Get::Show-Upgraded “true”;
.
Use the –no-act option to simulate actions without actually installing, removing, etc., any packages.
from: http://www.debian.org/doc/manuals/reference/ch-package.en.html
