Verify installed Debian package files against MD5 checksum lists from /var/lib/dpkg/info/*.md5sums. debsums can generate checksum lists from deb archives for packages that don’t include one.
debsums is intended primarily as a way of determining what installed files have been locally modified by the administrator or damaged by media errors and is of limited use as a security tool.
If you are looking for an integrity checker that can run from safe media, do integrity checks on checksum databases and can be easily configured to run periodically to warn the admin of changes see other tools such as: aide, integrit, samhain, or tripwire.
In the following example we will use debsums -ca List changed package files from all installed packages with checksums.
debsums -ce will list all the configuration files that has been changed since installation.