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