How to check for modified config files on a Debian system?

To find all Debian managed configuration files which have been changed from the default you can use a command like this.

dpkg-query -W -f="${Conffiles}\n" '*' | awk 'OFS="  "{print $2,$1}' | md5sum -c 2>/dev/null | awk -F': ' '$2 !~ /OK/{print $1}'

Edit (works with localized systems):

dpkg-query -W -f="${Conffiles}\n" '*' | awk 'OFS="  "{print $2,$1}' | LANG=C md5sum -c 2>/dev/null | awk -F': ' '$2 !~ /OK/{print $1}' | sort | less

Edit (works with packages with OK in the filename):

dpkg-query -W -f="${Conffiles}\n" '*' | awk 'OFS="  "{print $2,$1}' | LANG=C md5sum -c 2>/dev/null | awk -F': ' '$2 !~ /OK$/{print $1}' | sort | less