I suppose you could build a list of all packages that would be installed if you started with an empty apt database and installed everything that deborphan -a currently tells you. Then compare that with what's currently installed.
The difference would be packages that cannot be retrieved automatically with the current list of repositories.
I've no idea where to start with that though.
apt-show-versionsand should be merged into this older post instead. – Nov 20 '15 at 04:56