Reviewing upstream packages to write debian/copyright files is tedious manual work. It is done during initial packaging and after every new upstream release.
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.
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.
decopy generates debian/copyright files.
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