Find obsolete packages without using aptitude?

[Fredrik Jonson <fredrik@jonson.org>]

Hi All,

Here's a small challenge. I'm trying to find obsolete or orphaned packages
on a system that's been dist-upgraded. Aptitude can give me list with this
query:

 aptitude search ?obsolete

The definition of that query is: "This term matches any installed package
which is not available in any version from any archive.". It has been
installed locally or installed from a repository that is not in apt's
sources.list.

How would you accomplish that assuming you cannot use aptitude?

I've tried various arguments to dpkg-query, couldn't find the right
selector.  The closest I get is using 'deborphan --all-packages -p 5'. Not
quite though, deborphan spuriously also identifies a few packages that are
not relevant.

BTW, should I report the latter as a bug? F.x. deborphan reports 'chrony' as
orphaned, but I installed it yesterday and it sure looks available.

-- 
Fredrik Jonson