For tracking changes to your
/etc directory you can do as @Anthon has suggested and use git, subversion, mercurial, etc. to version control that directory. You can also use a tool such as etckeeper. There's a tutorial here as well as here.
etckeeper is a collection of tools to let /etc be stored in a git,
mercurial, bazaar or darcs repository. It hooks into apt to
automatically commit changes made to /etc during package upgrades. It
tracks file metadata that git does not normally support, but that is
important for /etc, such as the permissions of
quite modular and configurable, while also being simple to use if you
understand the basics of working with version control.
To my knowledge
apt does not have a way to check the files on disk vs. the files that are in the actual
.deb. Neither does
dpkg, the tool that
apt is actually using to do the management of files.
However you can use a tool such as
debsums to compare some of the files you have installed, it only looks at their checksums (md5sum) of what's in the
.deb file vs. what's on your systems disk.
See this serverfault question for more details about
dpkg checksumming, as well as this askubuntu question.
% debsums openssh-server