How do I find out where a package came from?

[David Goodenough <david.goodenough@btconnect.com>]

If I downloaded a package from one of the sources in my sources.list and did
not notice it at the time, how can I find out which source actually produced
this deb?

I have one of the KDE 3.1 sites in my sources.list, and I recently hit a 
problem with mpeglib which would not upgrade due to a file ownership
problem (/usr/bin/yaf-cdda) which package yaf provided and mpeglib wanted
to overwrite.  So I submitted a but to the BTS, and the maintainer came 
back with a somewhat cryptic one line which said that upgrading from
unofficial sources is not supported.  Now I do not know if yaf or mpeglib
is the offender, and I would like to be able to find out.

Is there an audit trail in apt-get?

David