Reviewing upstream packages to write debian/copyright files is tedious manual work. It tends not to get done again after initial packaging, especially not on every release (when something may have changed).
Making initial copyright file construction, and subsequent review/update easier will improve Debian's software quality.
Stretch (Debian 9) has significantly improved tools over previous releases.
licensecheck from devscripts can scan source code and report found copyright holders and known licenses.
scan-copyrights from libconfig-model-dpkg-perl can update an existing copyright file from rescanning the source. It can also create one from scratch. It uses licensecheck.
cme update dpkg-copyright
A script from cdbs can generate a copyright file using licensecheck:
licensecheck --copyright -r `find * -type f` | \ /usr/lib/cdbs/licensecheck2dep5 > debian/copyright.auto
license-reconcile compares the existing copyright with the source code and reports discrepancies.
debmake -k also compares the existing copyright with the source code and reports discrepancies.
debmake -cc generates a new copyright file from the source code.
licensee from ruby-licensee checks LICENSE files and returns known license names.
Some of the above are run by check-all-the-things -f copyright.
Updating debian copyright file with cme by Dominique Dumont
Creating, updating and checking debian/copyright semi-automatically by Petter Reinholdtsen