Using cfengine's own package tool

/var/lib/cfengine3/modules/packages/apt_get list-updates < /dev/null

/var/lib/cfengine3/modules/packages/apt_get list-updates-local < /dev/null

/var/lib/cfengine3/modules/packages/apt_get list-installed < /dev/null

Using Debian's own package tool

apt-get update (optional if cache is reasonably up-to-date)

apt-cache dump | fgrep "Package:"

apt-cache dumpavail | fgrep "Package:"